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-As filed with the Securities and Exchange Commission on October 19, 2018

Registration No. 333-[·]

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-4

REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

MIDWESTONE FINANCIAL GROUP, INC.
(Exact name of registrant as specified in its charter)

102 South Clinton Street

Iowa City, IA 52240

(319) 356-5800
(Address, Including Zip Code, and Telephone Number, Including 
Area Code, of Registrant’s Principal Executive Offices)

102 South Clinton Street

Iowa City, IA 52240

(319) 356-5800
(Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Copies to:

Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this registration statement becomes effective and upon completion of the Merger.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box: o

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. o

If applicable, place an x in the box to designate the appropriate rule provision relied upon in conducting this transaction:

(1)   Represents the estimated maximum number of shares to be issued to holders of ATBancorp common stock pursuant to the Agreement and Plan of Merger dated August 21, 2018, between the registrant and ATBancorp, which is attached to the joint proxy statement/prospectus as Appendix A.

(2)   Estimated solely for the purpose of calculating the registration fee pursuant to Rule 457(f)(2) and (f)(3) of Regulation C under the Securities Act of 1933, as amended.

(3)   Computed in accordance with Rule 457(f) under the Securities Act to be $13,477.60, which is equal to 0.0001212 multiplied by the proposed maximum aggregate offering price of $111,201,289.92.

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

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The information in this joint proxy statement/prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This joint proxy statement/prospectus is not an offer to sell these securities, and it is not soliciting to buy these securities, in any state where the offer or sale is not permitted.

Preliminary Proxy Statement/Prospectus

Subject to completion, dated October 19, 2018

Dear Shareholders of MidWestOne Financial Group, Inc. and ATBancorp:

We are pleased to report that the boards of directors of MidWestOne Financial Group, Inc., referred to as MidWestOne, and ATBancorp, together referred to as the parties, have approved a strategic merger involving our two companies (the “Merger”). We cannot complete the combination without your approval. If the Merger proposals described in this document are approved by the shareholders of each party and the Merger is subsequently completed, ATBancorp will merge with and into MidWestOne, with MidWestOne as the surviving entity.

Pursuant to the terms of an Agreement and Plan of Merger dated August 21, 2018 (the “Merger Agreement”), each share of ATBancorp common stock issued and outstanding immediately prior to the effective time of the Merger (the “Effective Time”), excluding any Cancelled Shares and any Dissenters’ Shares (each as defined in the Merger Agreement), shall represent the right to receive (i) 117.5500 fully paid and nonassessable shares of MidWestOne common stock (the “Per Share Stock Consideration”) and (ii) $992.51 (the “Per Share Cash Consideration”).

MidWestOne expects to issue approximately 4,117,541 shares of its common stock upon completion of the Merger.  MidWestOne common stock is traded on NASDAQ under the symbol “MOFG.” As reported on the NASDAQ, MidWestOne’s common stock closed at $33.25 on August 21, 2018, the day before the announcement of the Merger, and at $[·] on [·], 2018, the last practicable trading day before the date of this joint proxy statement/prospectus. Based on these closing prices, the transaction value is estimated at $171.67 million, or $4,901.05 per share of ATBancorp common stock, or $[·] million, or $[·] per share of ATBancorp common stock, respectively. Based on a Final Acquiror Market Value (as defined below) of MidWestOne’s common stock of $26.60 (the price beneath which ATBancorp would have the right to terminate the Merger Agreement, as described below, subject to the other conditions therein, including MidWestOne’s right to increase the amount of consideration payable to ATBancorp shareholders), the transaction value is estimated at $144.29 million, or $4,119.34 per share of ATBancorp common stock.  The ultimate value of the consideration received by ATBancorp shareholders will depend on the trading value of MidWestOne common stock prior to the closing of the Merger.

ATBancorp has the right to terminate the Merger Agreement upon a significant decrease in the price of MidWestOne’s common stock. In particular, ATBancorp may terminate the Merger Agreement if, as of the first date on which all requisite regulatory approvals (and waivers, if applicable) necessary for consummation of the Merger have been received (disregarding any waiting period) (the “Determination Date”): (i) the weighted average of the daily closing sales prices of a share of MidWestOne’s common stock as reported on the Nasdaq Global Select Market for the 20 consecutive trading days immediately preceding the Determination Date (the “Final Acquiror Market Value”) is less than $26.60; and (ii) the number obtained by dividing the Final Acquiror Market Value by $33.25 is less than the quotient of (x) the average of the daily closing value of the NASDAQ Bank Index for the 20 consecutive trading days immediately preceding the Determination Date, and (y) $4,315.63 (such quotient, the “Index Ratio”), multiplied by 0.80. If ATBancorp provides notice of such termination to MidWestOne, MidWestOne may elect to increase the Per Share Stock Consideration to equal the lesser of (1) a quotient, the numerator of which is equal to the product of (A) $33.25; (B) the Per Share Stock Consideration; and (C) the Index Ratio multiplied by 0.80, and the denominator of which is equal to the Final Acquiror Market Value; or (2) the quotient determined by dividing the $33.25 by the Final Acquiror Market Value, and multiplying the quotient by the product of the Per Share Stock Consideration and 0.80.

The effect of ATBancorp’s termination right and MidWestOne’s right to increase consideration is to ensure that the total consideration paid to ATBancorp shareholders is at least $144.29 million.

The Merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended, (the “Internal Revenue Code”). Assuming the Merger qualifies as a reorganization, a shareholder of ATBancorp generally will not recognize any gain or loss upon receipt of MidWestOne common stock in exchange for ATBancorp common stock in the Merger, and will recognize gain (but not loss) in an amount not to exceed any cash received as part of the Merger consideration (except with respect to any cash received in lieu of a fractional share of MidWestOne common stock, as discussed below under “Material U.S. Federal Income Tax Consequences of the Merger—Cash Received In Lieu of a Fractional Share of MidWestOne Common Stock”).

MidWestOne and ATBancorp will each hold a special meeting of shareholders to consider certain matters relating to the proposed Merger. MidWestOne and ATBancorp cannot complete the proposed Merger unless: (1) MidWestOne’s shareholders vote to approve and adopt the Merger Agreement and the transactions contemplated thereby and approve the issuance of MidWestOne common stock in connection with the Merger,

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and (2) ATBancorp’s shareholders vote to approve and adopt the Merger Agreement and the transactions contemplated thereby. The parties’ respective boards of directors are providing this document to solicit your proxy to vote for approval of the parties’ respective proposals.

This document is also being delivered to ATBancorp shareholders as MidWestOne’s prospectus for its offering of MidWestOne common stock in connection with the Merger.

ATBancorp shareholders will have the right to demand appraisal of their shares of ATBancorp common stock and obtain payment in cash for the fair value of their shares, but only if they perfect their appraisal rights and comply with the applicable provisions of Iowa law. For more information regarding dissenters’ rights, please see “Questions and Answers About the Merger and the Special Meetings—Are ATBancorp Shareholders Entitled to Appraisal Rights?” on page 7 and “The Merger—ATBancorp Shareholder Appraisal Rights” beginning on page 86.

Your vote is very important. To ensure your representation at the MidWestOne or ATBancorp special meeting, as applicable, please complete and return the enclosed proxy card or submit your proxy by one of the other means described below. Whether or not you expect to attend the MidWestOne or ATBancorp special meeting, as applicable, please vote promptly. Submitting a proxy now will not prevent you from being able to vote in person at the applicable special meeting. Each of the MidWestOne and ATBancorp boards of directors has approved the Merger Agreement and the transactions contemplated thereby and recommends to its shareholders to vote “FOR” adoption or approval of their respective proposals.

This document provides you with detailed information about the proposed Merger. It also contains or references information about MidWestOne and ATBancorp and certain related matters. You are encouraged to read this document carefully. In particular, you should read the “Risk Factors” section beginning on page 36 for a discussion of the risks you should consider in evaluating the proposed Merger and how it will affect you.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the Merger, the issuance of MidWestOne common stock in connection with the Merger or the other transactions described in this document, or passed upon the adequacy or accuracy of the disclosure in this document. Any representation to the contrary is a criminal offense.

The securities to be issued in connection with the Merger are not savings accounts, deposits or other obligations of any bank, non-bank subsidiary or savings association and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency. The securities are subject to investment risk, including possible loss of principal.

This document is dated [·], 2018 and is first being mailed to shareholders of MidWestOne and ATBancorp on or about [·], 2018.

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WHERE YOU CAN FIND MORE INFORMATION

MidWestOne files annual, quarterly and special reports, proxy statements and other business and financial information with the Securities and Exchange Commission (the “SEC”). You may read and copy any materials that MidWestOne files with the SEC at the SEC’s Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549, at prescribed rates. Please call the SEC at (800) SEC-0330 for further information on the public reference room. In addition, MidWestOne files reports and other business and financial information with the SEC electronically, and the SEC maintains a website located at http://www.sec.gov containing this information. You will also be able to obtain these documents, free of charge, from MidWestOne at www.midwestone.com under the “About MidWestOne Financial Group,” and the “SEC Filings” links.

MidWestOne has filed a registration statement on Form S-4 of which this document forms a part. As permitted by SEC rules, this document does not contain all of the information included in the registration statement or in the exhibits or schedules to the registration statement. You may read and copy the registration statement, including any amendments, schedules and exhibits, at the addresses set forth below. Statements contained in this document as to the contents of any contract or other documents referred to in this document are not necessarily complete. In each case, you should refer to the copy of the applicable contract or other document filed as an exhibit to the registration statement. This document incorporates by reference documents that MidWestOne has previously filed with the SEC. They contain important information about the company and its financial condition. For further information, please see the section entitled “Incorporation of Certain Documents by Reference” beginning on page 148. These documents are available without charge at its principal executive office, at the following address and telephone number:

MidWestOne Financial Group, Inc.

102 South Clinton Street

Iowa City, Iowa 52240

Attn: Kenneth R. Urmie, Corporate Secretary

(319) 356-5800

To obtain timely delivery of these documents, you must request the information no later than five business days before the date of the MidWestOne special meeting of shareholders in order to receive them before MidWestOne’s special meeting and no later than five business days before the date of the ATBancorp special meeting of shareholders in order to receive them before ATBancorp’s special meeting.

In addition, if you have questions about the Merger or the ATBancorp special meeting, need additional copies of this joint proxy statement/prospectus or need to obtain proxy cards or other information related to the proxy solicitation, you may contact ATBancorp at its principal executive office, at the following address and telephone number:

ATBancorp

895 Main Street

Dubuque, Iowa 52001

Attention: John W. Marshall, Secretary

(563) 589-7178

ATBancorp does not have a class of securities registered under Section 12 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), is not subject to the reporting requirements of Section 13(a) or 15(d) of the Exchange Act, and accordingly does not file documents or reports with the SEC.

You should rely only on the information contained or incorporated by reference in this joint proxy statement/prospectus. No one has been authorized to provide you with information that is different from what is contained in this joint proxy statement/prospectus. You should not assume that the information contained in this joint proxy statement/prospectus is accurate as of any date other than the date of this joint proxy statement/prospectus, and neither the mailing of this joint proxy statement/prospectus to MidWestOne and ATBancorp shareholders nor the issuance of MidWestOne common stock in the Merger shall create any implication to the contrary.

Information on the websites of MidWestOne or ATBancorp, or any subsidiary of MidWestOne or ATBancorp, is not part of this document or incorporated by reference herein. You should not rely on that information in deciding how to vote.

This document does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction. Except where the context otherwise indicates, information contained in this document regarding MidWestOne has been provided by MidWestOne and information contained in this document regarding ATBancorp has been provided by ATBancorp.

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102 South Clinton St.

Iowa City, Iowa 52240

(319) 356-5800

NOTICE OF THE SPECIAL MEETING OF SHAREHOLDERS

TO BE HELD ON [·]

NOTICE IS HEREBY GIVEN that a special meeting of the shareholders of MidWestOne Financial Group, Inc. (“MidWestOne”), will be held at [·], at [·], Central Time, on [·] for the following purposes:

1.              Approval and adoption of the Agreement and Plan of Merger, dated as of August 21, 2018, by and between MidWestOne Financial Group, Inc., or MidWestOne, and ATBancorp, as such agreement may be amended from time to time (the “Merger Agreement”), and the transactions contemplated thereby (the “MidWestOne merger proposal”);

2.              Approval of the issuance of MidWestOne common stock in the Merger (the “MidWestOne stock issuance proposal”); and

3.              Approval of one or more adjournments of the MidWestOne special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the MidWestOne merger proposal and the MidWestOne stock issuance proposal (the “MidWestOne adjournment proposal”).

MidWestOne will transact no other business at the special meeting, except for business properly brought before the special meeting or any adjournment or postponement thereof.

The above proposals are described in more detail in this document, which you should read carefully in its entirety before you vote. A copy of the Merger Agreement is attached as Appendix A to this document.

The MidWestOne board of directors has set [·] as the record date for the MidWestOne special meeting. Only holders of record of MidWestOne common stock at the close of business on [·] will be entitled to notice of and to vote at the MidWestOne special meeting and any adjournments or postponements thereof. Any shareholder entitled to attend and vote at the MidWestOne special meeting is entitled to appoint a proxy to attend and vote on such shareholder’s behalf. Such proxy need not be a holder of MidWestOne common stock.

Your vote is very important. To ensure your representation at the MidWestOne special meeting, please complete and return the enclosed proxy card or submit your proxy by telephone, through the Internet or by mail. Please vote promptly whether or not you expect to attend the MidWestOne special meeting. Submitting a proxy now will not prevent you from being able to vote in person at the MidWestOne special meeting.

The MidWestOne board of directors has approved the Merger Agreement and the transactions contemplated thereby and recommends that you vote “FOR” the MidWestOne merger proposal, “FOR” the MidWestOne stock issuance proposal and “FOR” the MidWestOne adjournment proposal (if necessary or appropriate).

Iowa City, Iowa

[·], 2018

PLEASE VOTE YOUR SHARES OF MIDWESTONE COMMON STOCK PROMPTLY. YOU CAN FIND INSTRUCTIONS FOR VOTING ON THE ENCLOSED PROXY CARD. IF YOU HAVE QUESTIONS ABOUT THE PROPOSALS OR ABOUT VOTING YOUR SHARES, PLEASE CALL MIDWESTONE INVESTOR RELATIONS AT (319) 356-5800.

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ATBANCORP

895 MAIN STREET

DUBUQUE, IOWA 52001

(563) 582-1841

NOTICE OF THE SPECIAL MEETING OF SHAREHOLDERS

TO BE HELD ON [·]

To the holders of ATBancorp common stock:

NOTICE IS HEREBY GIVEN that a special meeting of the shareholders of ATBancorp will be held at [·], at [·], Central Time, on [·] for the following purposes:

1.              Approval and adoption of the Agreement and Plan of Merger, dated as of August 21, 2018, by and between MidWestOne Financial Group, Inc. and ATBancorp, as such agreement may be amended from time to time (the “Merger Agreement”), and the transactions contemplated thereby (the “ATBancorp merger proposal”); and

2.              Approval of one or more adjournments of the ATBancorp special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the ATBancorp merger proposal (the “ATBancorp adjournment proposal”).

ATBancorp will transact no other business at the special meeting, except for business properly brought before the special meeting or any adjournment or postponement thereof.

The above proposals are described in more detail in this document, which you should read carefully in its entirety before you vote. A copy of the Merger Agreement is attached as Appendix A to this document.

The ATBancorp board of directors has set [·] as the record date for the ATBancorp special meeting. Only holders of record of ATBancorp common stock at the close of business on [·] will be entitled to notice of and to vote at the ATBancorp special meeting and any adjournments or postponements thereof. Any shareholder entitled to attend and vote at the ATBancorp special meeting is entitled to appoint a proxy to attend and vote on such shareholder’s behalf. Such proxy need not be a holder of ATBancorp common stock.

Your vote is very important. To ensure your representation at the ATBancorp special meeting, please complete and return the enclosed proxy card. Please vote promptly whether or not you expect to attend the ATBancorp special meeting. Submitting a proxy now will not prevent you from being able to vote in person at the ATBancorp special meeting.

The ATBancorp board of directors has approved the Merger Agreement and the transactions contemplated thereby and recommends that you vote “FOR” the ATBancorp merger proposal and “FOR” the ATBancorp adjournment proposal (if necessary or appropriate).

Dubuque, Iowa
[·], 2018

PLEASE VOTE YOUR SHARES OF ATBANCORP COMMON STOCK PROMPTLY. YOU CAN FIND INSTRUCTIONS FOR VOTING ON THE ENCLOSED PROXY CARD. IF YOU HAVE QUESTIONS ABOUT THE PROPOSALS OR ABOUT VOTING YOUR SHARES, PLEASE CALL JOHN W. MARSHALL, SECRETARY, AT (563) 589-7178.

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QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE SPECIAL MEETINGS

The following are answers to certain questions that you may have regarding the special meetings. The parties urge you to read carefully the remainder of this document because the information in this section may not provide all the information that might be important to you in determining how to vote. Additional important information is also contained in the appendices to, and the documents incorporated by reference in, this document.

Q:                                  WHAT IS THE MERGER?

A.                                    MidWestOne and ATBancorp have entered into a Merger Agreement pursuant to which, subject to the terms and conditions of the Merger Agreement, ATBancorp will merge with and into MidWestOne, with MidWestOne continuing as the surviving corporation (the “Merger”). A copy of the Merger Agreement is attached as Appendix A to this document. Immediately following the Merger, each of American Trust & Savings Bank (“ATSB”), an Iowa state chartered bank, and American Bank & Trust Wisconsin (“ABTW”), a Wisconsin state chartered bank, each of which is a wholly-owned subsidiary of ATBancorp, will merge with and into MidWestOne Bank, an Iowa state chartered bank and a wholly owned subsidiary of MidWestOne (“MidWestOne Bank”), with MidWestOne Bank continuing as the surviving bank (the “Bank Mergers”). In order to complete the transaction, each party needs not only the approval of its respective shareholders but the approval of each of these mergers by the applicable banking regulators of MidWestOne, MidWestOne Bank, ATBancorp, ATSB and ABTW.

Q:                                  WHY AM I RECEIVING THIS JOINT PROXY STATEMENT/PROSPECTUS?

A.                                    Each of MidWestOne and ATBancorp is sending these materials to its shareholders to help them decide how to vote their shares of MidWestOne or ATBancorp common stock, as the case may be, with respect to the matters to be considered at the special meetings.

The Merger cannot be completed unless MidWestOne shareholders approve and adopt the Merger Agreement and the transactions contemplated thereby and approve the issuance of MidWestOne common stock in the Merger, and ATBancorp shareholders approve and adopt the Merger Agreement and the transactions contemplated thereby. Each of MidWestOne and ATBancorp is holding a special meeting of its shareholders to vote on the proposals necessary to complete the Merger. Information about these special meetings, the Merger and the business to be considered by shareholders at each of the special meetings is contained in this document.

This document constitutes both a joint proxy statement of MidWestOne and ATBancorp and a prospectus of MidWestOne. It is a joint proxy statement because each of the boards of directors of MidWestOne and ATBancorp is soliciting proxies using this document from their respective shareholders. It is a prospectus because MidWestOne, in connection with the Merger, is offering shares of its common stock in exchange for outstanding shares of ATBancorp common stock.

Q:                                  WHAT WILL ATBANCORP SHAREHOLDERS RECEIVE IN THE MERGER?

A:                                   Each share of ATBancorp common stock issued and outstanding immediately prior to the effective time of the Merger (“Effective Time”), excluding any Cancelled Shares and any Dissenters’ Shares (each as defined in the Merger Agreement), shall represent the right to receive (i) the Per Share Stock Consideration and (ii) the Per Share Cash Consideration.

For each fractional share that would otherwise be issued, MidWestOne will pay cash in an amount equal to such fraction multiplied by the volume weighted average of the daily closing sales prices of a share of MidWestOne’s common stock as reported on the Nasdaq Global Select Market for the 20 consecutive trading days immediately preceding the Closing Date (the “Closing Acquiror Common Stock Price”). The “Closing Date” will be the first day of the month immediately following the month in which the satisfaction or waiver of the latest to occur of MidWestOne’s and ATBancorp’s respective closing conditions occurs, or at such other time and place as MidWestOne and ATBancorp may agree. No interest will be paid or accrue on cash payable to holders in lieu of fractional shares.

Based on MidWestOne’s common stock’s closing price of $33.25 on August 21, 2018, the day before the announcement of the Merger, the transaction value is estimated at $171.67 million, or $4,901.05 per share of ATBancorp common stock. Based on MidWestOne’s common stock’s closing price of $[·] on [·], 2018, the transaction value is estimated at $[·] million, or $[·] per share of ATBancorp common stock. Based on a Final Acquiror Market Value of MidWestOne’s closing stock of $26.60 (the price beneath which ATBancorp would have the right to terminate the Merger Agreement, subject to the other conditions therein, including MidWestOne’s right to increase the amount of consideration payable to ATBancorp shareholders), the transaction value is estimated at $144.29 million, or $4,119.34 per share of ATBancorp common stock.

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The ultimate value of the consideration received by ATBancorp shareholders will depend on the trading value of MidWestOne common stock prior to the closing of the Merger.

Q:                                  WILL THE VALUE OF THE MERGER CONSIDERATION CHANGE BETWEEN THE DATE OF THIS DOCUMENT AND THE TIME THE MERGER IS COMPLETED?

A:                                   Yes. The trading price of MidWestOne common stock may increase or decrease prior to the Closing Date, and the value of the shares of MidWestOne common stock that you would receive in connection with the Merger would increase or decrease accordingly. In addition, the number of shares of MidWestOne common stock that you may receive in the Merger may be increased in certain circumstances pursuant to the terms of the Merger Agreement. See “The Merger Agreement— Termination of the Merger Agreement.”

Q:                                  WILL ATBANCORP SHAREHOLDERS BE ABLE TO TRADE THE SHARES OF MIDWESTONE COMMON STOCK RECEIVED IN THE MERGER?

A:                                   Yes. The MidWestOne common stock issued in the Merger to ATBancorp shareholders, except for holders of any Dissenters’ Shares, will be registered under the Securities Act of 1933, as amended (the “Securities Act”), and will be listed on NASDAQ under the symbol “MOFG.” All shares of MidWestOne common stock issued in the Merger will be freely transferable and will not be subject to any restrictions on transfer arising under the Securities Act, except for (i) shares issued to any ATBancorp shareholder who may be deemed to be an “affiliate” of MidWestOne after completion of the Merger, or (ii) shares subject to a Lock-Up Agreement described below. An affiliate of a corporation, as defined by the rules promulgated under the Securities Act, is a person who directly or indirectly, through one or more intermediaries, controls, is controlled by, or is under common control with, that corporation. Affiliates generally include directors, executive officers and beneficial owners of 10% or more of a corporation’s capital stock.

In connection with entering into the Merger Agreement, MidWestOne entered into Lock-Up Agreements (“Lock-Up Agreements”) with certain holders of ATBancorp common stock. The shareholders that are party to the Lock-Up Agreements beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock. The Lock-Up Agreements require that the shareholders that are parties thereto, during the period commencing as of the date on which the Effective Time occurs and expiring 180 days thereafter, not directly or indirectly, with certain exceptions, take any action to: (i) offer, sell, contract to sell, sell any option, warrant or contract to purchase, purchase any option, warrant or contract to sell, transfer, pledge or otherwise dispose of, or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition or otherwise) of any shares of MidWestOne common stock whether owned as of the time of the Lock-Up Agreements or subsequently acquired, including shares of MidWestOne common stock received by the shareholder pursuant to the Merger Agreement; (ii) enter into any swap or other derivative transaction that transfers to another, in whole or in part, any of the economic benefits or risks of ownership of MidWestOne common stock, whether any such transaction is to be settled by delivery of MidWestOne common stock or other securities, in cash or otherwise; or (iii) publicly disclose an intention to effect any transaction contemplated by clause (i) or (ii).  If the Merger Agreement is terminated without the consummation of the Merger, the Lock-Up Agreements will automatically terminate.

Former ATBancorp shareholders who are not affiliates of MidWestOne after the completion of the Merger and are not subject to the terms of a Lock-Up Agreement may sell their shares of MidWestOne common stock received in the Merger at any time. Former ATBancorp shareholders who become affiliates of MidWestOne after completion of the Merger and are not subject to the terms of a Lock-Up Agreement will be subject to the volume and sale limitations of Rule 144 under the Securities Act until they are no longer affiliates of MidWestOne. This joint proxy statement/prospectus does not cover resales of MidWestOne common stock received by any person upon completion of the Merger, and no person is authorized to make any use of or rely on this joint proxy statement/prospectus in connection with or to effect any resale of MidWestOne common stock.

Q:                                  WHAT ARE THE SPECIAL DIVIDENDS?

A:                                   The Merger Agreement provides that ATBancorp may pay a special dividend of $907.8452 per share to the ATBancorp shareholders at any time prior to the closing of the Merger (the “Closing”) as determined by ATBancorp.  Such dividend represents a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received as consideration in connection with the sale of United American Bank.  The special dividend was distributed to ATBancorp shareholders on September 24, 2018.

The Merger Agreement also provides that ATBancorp may pay a special dividend to shareholders based on the net proceeds (if any) received by ATBancorp in connection with the disposition by ATBancorp of certain of its businesses—American

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Trust Retirement, ATBancorp’s California trust office, and ATInsurance—as well as 100% of the equity of ATCapital Management, a registered investment advisor and a wholly owned subsidiary of ATBancorp (collectively, the “ATB Businesses”), prior to Closing, as required by the Merger Agreement. The payment of these special dividends by ATBancorp is in addition to the Merger consideration to be paid by MidWestOne. ATBancorp shareholders are not required to take any action to approve or receive these special dividends.

Q:                                  WHEN WILL THE MERGER BE COMPLETED?

A:                                   MidWestOne and ATBancorp are working to complete the Merger as soon as practicable. The Merger will occur on the first day of the month immediately following the month in which the satisfaction or waiver of the latest to occur of the closing conditions set forth in the Merger Agreement or at such other time and place as MidWestOne and ATBancorp may agree. Neither MidWestOne nor ATBancorp can predict the actual date on which the Merger will be completed because it is subject to factors beyond each company’s control, including whether or when the parties’ respective shareholders’ approvals will be received. For further information, please see the section entitled “The Merger Agreement—Conditions to Consummation of the Merger” beginning on page 97.

Q:                                  WHO IS ENTITLED TO VOTE?

A:                                   MidWestOne Special Meeting.  Holders of record of MidWestOne common stock at the close of business on [·], 2018, which is the date that the MidWestOne board of directors has fixed as the record date for the MidWestOne special meeting, are entitled to vote at the MidWestOne special meeting.

ATBancorp Special Meeting.  Holders of record of ATBancorp common stock at the close of business on [·], 2018, which is the date that the ATBancorp board of directors has fixed as the record date for the ATBancorp special meeting, are entitled to vote at the ATBancorp special meeting.

Q:                                  WHAT CONSTITUTES A QUORUM?

A:                                   MidWestOne Special Meeting.  A majority of the outstanding shares of MidWestOne entitled to vote, represented in person or by proxy, will constitute a quorum for the transaction of business at the MidWestOne special meeting. Abstentions and broker non-votes, if any, will be included in determining the number of shares present at the meeting for the purpose of determining the presence of a quorum.

ATBancorp Special Meeting.  A majority of the outstanding shares of ATBancorp entitled to vote, represented in person or by proxy, will constitute a quorum for the transaction of business at the ATBancorp special meeting. Abstentions will be included in determining the number of shares present at the meeting for the purpose of determining the presence of a quorum.

Q:                                  WHAT AM I BEING ASKED TO VOTE ON AND WHY IS THIS APPROVAL NECESSARY?

A:                                   MidWestOne Special Meeting.  MidWestOne shareholders are being asked to vote on the following proposals:

1.              To approve the MidWestOne merger proposal.

2.              To approve the MidWestOne stock issuance proposal.

3.              To approve the MidWestOne adjournment proposal (if necessary or appropriate).

Shareholder approval of the MidWestOne merger proposal and the MidWestOne stock issuance proposal is required to complete the Merger. MidWestOne will transact no business other than as listed above at the MidWestOne special meeting, except for business properly brought before the MidWestOne special meeting or any adjournment or postponement thereof.

ATBancorp Special Meeting.  ATBancorp shareholders are being asked to vote on the following proposals:

1.              To approve the ATBancorp merger proposal.

2.              To approve the ATBancorp adjournment proposal (if necessary or appropriate).

Shareholder approval of the ATBancorp merger proposal is required to complete the Merger. ATBancorp will transact no business other than as listed above at the ATBancorp special meeting, except for business properly brought before the ATBancorp special meeting or any adjournment or postponement thereof.

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Q:                                  WHAT VOTE IS REQUIRED TO APPROVE EACH PROPOSAL AT THE MIDWESTONE SPECIAL MEETING?

A:                                   The MidWestOne merger proposal: The affirmative vote of a majority of the votes entitled to be cast on the proposal is required to approve the MidWestOne merger proposal.

The MidWestOne stock issuance proposal: The affirmative vote of a majority of the shares of MidWestOne common stock represented in person or by proxy at the MidWestOne special meeting and entitled to vote on the proposal is required to approve the MidWestOne stock issuance proposal.

The MidWestOne adjournment proposal: The affirmative vote of a majority of the shares of MidWestOne common stock represented in person or by proxy at the MidWestOne special meeting and entitled to vote on the proposal is required to approve the MidWestOne adjournment proposal.

Q:                                  WHAT DOES THE MIDWESTONE BOARD OF DIRECTORS RECOMMEND?

A:                                   The MidWestOne board of directors recommends that MidWestOne shareholders vote “FOR” the MidWestOne merger proposal, “FOR” the MidWestOne stock issuance proposal and “FOR” the MidWestOne adjournment proposal (if necessary or appropriate).

Q:                                  WHAT VOTE IS REQUIRED TO APPROVE EACH PROPOSAL AT THE ATBANCORP SPECIAL MEETING?

A:                                   The ATBancorp merger proposal:  The affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on the proposal is required to approve the ATBancorp merger proposal.  Pursuant to the Voting Agreement (as defined below), approval of the ATBancorp merger proposal is virtually assured.

The ATBancorp adjournment proposal:  The affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on the proposal is required to approve the ATBancorp adjournment proposal.

Q:                                  WHAT DOES THE ATBANCORP BOARD OF DIRECTORS RECOMMEND?

A:                                   The ATBancorp board of directors recommends that ATBancorp shareholders vote “FOR” the ATBancorp merger proposal and “FOR” the ATBancorp adjournment proposal (if necessary or appropriate).

Q:                                  WHAT DO I NEED TO DO NOW?

A:                                   After carefully reading and considering the information contained in this document, please vote your shares as soon as possible so that your shares will be represented at your respective company’s special meeting. Please follow the instructions set forth on the proxy card or on the voting instruction form provided by the record holder if your shares are held in the name of your broker, bank or other nominee.

Q:                                  HOW DO I VOTE?

A:                                   MidWestOne Special Meeting.  If you are a shareholder of record of MidWestOne as of the MidWestOne record date, you may submit your proxy before MidWestOne’s special meeting in one of the following ways:

·                  use the toll-free number shown on your proxy card;

·                  visit the website shown on your proxy card to vote via the Internet; or

·                  complete, sign, date and return the enclosed proxy card in the enclosed postage-paid envelope.

You may also cast your vote in person at the MidWestOne special meeting.

If your shares are held in “street name,” through a broker, bank or other nominee, that institution will send you separate instructions describing the procedure for voting your shares. “Street name” shareholders who wish to vote at the meeting will need to obtain a proxy form from their broker, bank or other nominee.

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ATBancorp Special Meeting.  If you are a shareholder of ATBancorp as of the ATBancorp record date, you may submit your proxy before ATBancorp’s special meeting by completing, signing, dating and returning the enclosed proxy card in the enclosed postage-paid envelope.

You may also cast your vote in person at the ATBancorp special meeting.

Q:                                  HOW MANY VOTES DO I HAVE?

A:                                   MidWestOne Shareholders.  You are entitled to one vote for each share of MidWestOne common stock that you owned as of the MidWestOne record date. As of the close of business on the MidWestOne record date, there were approximately [·] outstanding shares of MidWestOne common stock entitled to vote. As of that date, approximately [·]% of such outstanding shares of MidWestOne common stock were beneficially owned by the directors and executive officers of MidWestOne and their respective affiliates.

ATBancorp Shareholders.  You are entitled to one vote for each share of ATBancorp common stock that you owned as of the ATBancorp record date. As of the close of business on the ATBancorp record date, there were 35,028 outstanding shares of ATBancorp common stock entitled to vote. As of that date, approximately [·]% of such outstanding shares of ATBancorp common stock were beneficially owned by the directors and executive officers of ATBancorp and their respective affiliates. In addition, shareholders owning 62.41% of the outstanding shares of ATBancorp common stock are parties to a voting and support agreement with MidWestOne (the “Voting Agreement”) requiring them to vote in favor of the ATBancorp merger proposal.

Q:                                  WHEN AND WHERE ARE THE MIDWESTONE AND ATBANCORP SPECIAL MEETINGS?

A:                                   The special meeting of MidWestOne shareholders will be held at [·] at [·] [·].m., Central Time, on [·], [·], 2018. Subject to space availability, all MidWestOne shareholders as of the MidWestOne record date, or their duly appointed proxies, may attend the MidWestOne special meeting.

The special meeting of ATBancorp shareholders will be held at [·] at [·] [·].m., Central Time, on [·], [·], 2018. All ATBancorp shareholders as of the ATBancorp record date, or their duly appointed proxies, may attend the ATBancorp special meeting.

Q:                                  IF MY MIDWESTONE SHARES ARE HELD IN “STREET NAME” BY A BROKER, BANK OR OTHER NOMINEE, WILL MY BROKER, BANK OR OTHER NOMINEE VOTE MY SHARES FOR ME?

A:                                   If you are a MidWestOne shareholder and your shares are held in “street name” in a stock brokerage account or by a bank or other nominee, you must provide the record holder of your shares with instructions on how to vote your shares. Please follow the voting instructions provided by your broker, bank or other nominee. Please note that you may not vote shares held in street name by returning a proxy card directly to MidWestOne or by voting in person at the MidWestOne special meeting unless you provide a “legal proxy,” which you must obtain from your broker, bank or other nominee.

Under the rules of NASDAQ, brokers who hold shares in street name for a beneficial owner of those shares typically have the authority to vote in their discretion on “routine” proposals when they have not received instructions from beneficial owners. However, brokers are not allowed to exercise their voting discretion with respect to the approval of matters that NASDAQ determines to be “non-routine” without specific instructions from the beneficial owner. It is expected that all proposals to be voted on at the MidWestOne special meeting will be “non-routine” matters. Broker non-votes occur when a broker or nominee is not instructed by the beneficial owner of shares to vote on a particular proposal for which the broker does not have discretionary voting power.

If you are a MidWestOne shareholder and you hold your shares in street name and you do not instruct your broker, bank or other nominee on how to vote your shares:

·                  your broker, bank or other nominee may not vote your shares on the MidWestOne merger proposal, which broker non-votes will have no effect on the vote count for such proposal;

·                  your broker, bank or other nominee may not vote your shares on the MidWestOne stock issuance proposal, which broker non-votes will have no effect on the vote count for such proposal; and

·                  your broker, bank or other nominee may not vote your shares on the MidWestOne adjournment proposal, which broker non-votes will have no effect on the vote count for such proposal.

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Q:                                  HOW WILL MY MIDWESTONE SHARES HELD IN THE EMPLOYEE STOCK OWNERSHIP PLAN BE VOTED?

A:                                   MidWestOne maintains an employee stock ownership plan (“ESOP”) that as of the MidWestOne record date owns [·] or [·]% of the outstanding shares of MidWestOne’s common stock. Employees of MidWestOne and MidWestOne Bank participate in the ESOP. As of the MidWestOne record date, [·] shares have been allocated to ESOP participants. Each ESOP participant has the right to instruct the trustee of the plan how to vote the shares of MidWestOne common stock allocated to his or her account under the ESOP. If an ESOP participant properly executes the voting instruction card, the ESOP trustee will vote the participant’s shares in accordance with the participant’s instructions. Shares of MidWestOne’s common stock held in the ESOP, but not allocated to any participant’s account, and allocated shares for which no voting instructions are received from participants, will be voted by the trustee in proportion to the results of the votes cast on the issue by the participants and beneficiaries.

Q:                                  WHAT IF I DO NOT VOTE OR I ABSTAIN?

A:                                   For purposes of each of the MidWestOne special meeting and the ATBancorp special meeting, an abstention occurs when a shareholder attends the applicable special meeting, either in person or represented by proxy, but abstains from voting.

For the MidWestOne merger proposal, if a MidWestOne shareholder present in person at the MidWestOne special meeting abstains from voting, or responds by proxy with an “abstain” vote, it will have the same effect as a vote cast “AGAINST” this proposal. If a MidWestOne shareholder is not present in person at the MidWestOne special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for such proposal.

For the MidWestOne stock issuance proposal and the MidWestOne adjournment proposal, if a MidWestOne shareholder present in person at the MidWestOne special meeting abstains from voting, or responds by proxy with an “abstain” vote, it will have the same effect as a vote cast “AGAINST” each such proposal. If a MidWestOne shareholder is not present in person at the MidWestOne special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for each such proposal.

For the ATBancorp merger proposal, if an ATBancorp shareholder present in person at the ATBancorp special meeting abstains from voting, or responds by proxy with an “abstain” vote, it will have the same effect as a vote cast “AGAINST” this proposal. If an ATBancorp shareholder is not present in person at the ATBancorp special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for this proposal.

For the ATBancorp adjournment proposal, if an ATBancorp shareholder present in person at the ATBancorp special meeting abstains from voting, or responds by proxy with an “abstain” vote, it will have the same effect as a vote cast “AGAINST” this proposal. If an ATBancorp shareholder is not present in person at the ATBancorp special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for this proposal.

Q:                                  WHAT WILL HAPPEN IF I RETURN MY PROXY OR VOTING INSTRUCTION CARD WITHOUT INDICATING HOW TO VOTE?

A:                                   If you sign and return your proxy or voting instruction card without indicating how to vote on any particular proposal, the MidWestOne common stock represented by your proxy will be voted as recommended by the MidWestOne board of directors with respect to each MidWestOne proposal and the ATBancorp common stock represented by your proxy will be voted as recommended by the ATBancorp board of directors with respect to each ATBancorp proposal. Unless a MidWestOne shareholder or an ATBancorp shareholder, as applicable, checks the box on its proxy card to withhold discretionary authority, the proxyholders may use their discretion to vote on other matters relating to the MidWestOne special meeting or ATBancorp special meeting, as applicable.

Q:                                  MAY I CHANGE MY VOTE AFTER I HAVE DELIVERED MY PROXY OR VOTING INSTRUCTION CARD?

A:                                   MidWestOne Shareholders.  Yes. You may change your vote at any time before your proxy is voted at the MidWestOne special meeting. You may do this in one of four ways:

·                  by sending a notice of revocation to the corporate secretary of MidWestOne;

·                  by logging onto the Internet website specified on your proxy card in the same manner you would to submit your proxy electronically or by calling the telephone number specified on your proxy card, in each case if you are eligible to do so and following the instructions on the proxy card;

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·                  by sending a completed proxy card bearing a later date than your original proxy card; or

·                  by attending the MidWestOne special meeting and voting in person.

If you choose any of the first three methods, you must take the described action such that the notice, internet vote or proxy card, as applicable, is received no later than the beginning of the MidWestOne special meeting.

If your shares are held in an account at a broker, bank or other nominee, you should contact your broker, bank or other nominee to change your vote.

ATBancorp Shareholders.  Yes. You may change your vote at any time before your proxy is voted at the ATBancorp special meeting. You may do this in one of three ways:

·                  by sending a notice of revocation to the corporate secretary of ATBancorp;

·                  by sending a completed proxy card bearing a later date than your original proxy card; or

·                  by attending the ATBancorp special meeting and voting in person.

If you choose either of the first two methods, you must take the described action such that the notice or proxy card, as applicable, is received no later than the beginning of the ATBancorp special meeting.

Q:                                  DO I NEED IDENTIFICATION TO ATTEND THE MIDWESTONE MEETING IN PERSON?

A:                                   Yes. Please bring proper identification, together with proof that you are a record owner of MidWestOne. If your shares of MidWestOne common stock are held in street name, please bring acceptable proof of ownership, such as a letter from your broker or an account statement showing that you beneficially owned shares of MidWestOne common stock on the record date.

Q:                                  ARE ATBANCORP SHAREHOLDERS ENTITLED TO APPRAISAL RIGHTS?

A:                                   Yes. ATBancorp shareholders will have the right to assert appraisal of their shares of ATBancorp common stock and obtain payment in cash for the fair value of their shares, but only if they perfect their dissenters’ rights and comply with the applicable provisions of Iowa law. A copy of the Iowa statutory provisions related to appraisal rights is attached as Appendix B to this document, and a summary of these provisions can be found under “The Merger—ATBancorp Shareholder Appraisal Rights” beginning on page 86. Due to the complexity of the procedures for exercising the right to seek appraisal, ATBancorp shareholders who are considering exercising such rights are encouraged to seek the advice of legal counsel. Failure to strictly comply with the applicable Iowa law provisions will result in the loss of the right of appraisal.

Q:                                  WHAT ARE THE MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER TO ATBANCORP SHAREHOLDERS?

A:                                   The Merger is intended to qualify, and the obligation of each party to complete the Merger is conditioned upon the party’s receipt of an opinion to the effect that the Merger will qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

Accordingly, ATBancorp shareholders are not expected to recognize any gain or loss upon receipt of MidWestOne common stock in exchange for ATBancorp common stock in the Merger, and will recognize gain (but not loss) in an amount not to exceed any cash received as part of the Merger consideration (except with respect to any cash received in lieu of a fractional share of MidWestOne common stock, as discussed below under “Material U.S. Federal Income Tax Consequences of the Merger—Cash Received In Lieu of a Fractional Share of MidWestOne Common Stock”). For a more detailed discussion of the material U.S. federal income tax consequences of the transaction, please see the section entitled “Material U.S. Federal Income Tax Consequences of the Merger” beginning on page 100.

The tax consequences of the Merger to any particular shareholder will depend on that shareholder’s particular facts and circumstances. Accordingly, you are urged to consult your tax advisor to determine your tax consequences from the Merger.

Q:                                  WHAT HAPPENS IF THE MERGER IS NOT COMPLETED?

A:                                   If the Merger Agreement is terminated and the Merger is not completed, ATBancorp shareholders will not receive any consideration for their shares of ATBancorp common stock that otherwise would have been received in connection with the Merger. Instead, ATBancorp will remain an independent company. In addition, if the Merger Agreement is terminated under certain circumstances, ATBancorp would be required to pay MidWestOne a termination fee of $7.6 million, plus reimburse MidWestOne’s expenses up to $1,000,000.

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Q:                                  SHOULD ATBANCORP SHAREHOLDERS SEND IN THEIR STOCK CERTIFICATES NOW?

A:                                   No. ATBancorp shareholders should not send in any stock certificates now. If the Merger is approved, transmittal materials with instructions for their completion will be provided to ATBancorp shareholders under separate cover and the stock certificates should be sent at that time.

Q:                                  WHAT SHOULD I DO IF I RECEIVE MORE THAN ONE SET OF VOTING MATERIALS?

A:                                   ATBancorp and MidWestOne shareholders may receive more than one set of voting materials, including multiple copies of this document and multiple proxy cards or voting instruction cards. For example, if you hold shares of MidWestOne common stock in more than one brokerage account, you will receive a separate voting instruction card for each brokerage account in which you hold such shares. If you are a holder of record of ATBancorp common stock or MidWestOne common stock and your shares are registered in more than one name, you will receive one or more separate proxy cards or voting instruction cards for each company. Please complete, sign, date and return each proxy card and voting instruction card that you receive or otherwise follow the voting instructions set forth in this document to ensure that you vote every share of ATBancorp common stock or MidWestOne common stock that you own.

Q:                                  WHO SHOULD I CONTACT IF I HAVE ANY QUESTIONS ABOUT THE PROXY MATERIALS OR VOTING?

A:                                   If you are a MidWestOne shareholder and have any questions about the proxy materials or if you need assistance submitting your proxy or voting your shares or need additional copies of this document or the enclosed proxy card, you should contact MidWestOne Investor Relations at (319) 356-5800.

If you are an ATBancorp shareholder and have any questions about the proxy materials or if you need assistance submitting your proxy or voting your shares or need additional copies of this document or the enclosed proxy card, you should contact John W. Marshall, Secretary, at (563) 589-7178.

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SUMMARY

This summary highlights selected information included in this document and does not contain all of the information that may be important to you. You should read this entire document and its appendices and the other documents to which the parties refer before you decide how to vote with respect to the proposals. In addition, the parties incorporate by reference important business and financial information about MidWestOne into this document. For a description of this information, please see the section entitled “Incorporation of Certain Documents by Reference” beginning on page 148. You may obtain the information incorporated by reference into this document without charge by following the instructions in the section entitled “Where You Can Find More Information” in the forepart of this document. Each item in this summary includes a page reference directing you to a more complete description of that item.

Unless the context otherwise requires, throughout this document, “MidWestOne” refers to MidWestOne Financial Group, Inc., “ATBancorp” refers to ATBancorp and “we,” “us” and “our” refer collectively to MidWestOne and ATBancorp. Also, the parties refer to the proposed merger of ATBancorp with and into MidWestOne as the “Merger,” the proposed merger of American Trust & Savings Bank and American Bank & Trust Wisconsin with MidWestOne Bank collectively as the “Bank Mergers” and the Agreement and Plan of Merger dated as of August 21, 2018, by and between MidWestOne and ATBancorp as the “Merger Agreement.”

The Merger and the Merger Agreement (pages 52 and 89)

The terms and conditions of the Merger are contained in the Merger Agreement, which is attached to this document as Appendix A. The parties encourage you to read the Merger Agreement carefully, as it is the legal document that governs the Merger. Under the terms of the Merger Agreement, ATBancorp will merge with and into MidWestOne with MidWestOne as the surviving corporation.

Merger Consideration (page 90)

In the Merger, each share of ATBancorp common stock issued and outstanding immediately prior to the Effective Time, excluding any Cancelled Shares and any Dissenters’ Shares, shall represent the right to receive without interest (i) 117.5500 shares of MidWestOne common stock (the “Per Share Stock Consideration”) and (ii) $992.51 (the “Per Share Cash Consideration”).

No fractional shares of MidWestOne common stock will be issued to any shareholder of ATBancorp upon completion of the Merger.  For each fractional share that would otherwise be issued, MidWestOne will pay an amount in cash (without interest) rounded to the nearest whole cent, determined by multiplying the Closing Acquiror Common Stock Price by the fractional share of MidWestOne common stock to which such former holder would otherwise be entitled.  No interest will be paid or accrue on cash payable to holders in lieu of fractional shares.

The ultimate value of the consideration received by ATBancorp shareholders will depend on the trading value of MidWestOne common stock prior to the closing of the Merger.  By way of example, the following MidWestOne closing prices would result in the corresponding total Merger consideration and consideration per share of ATBancorp common stock:

It is currently expected that the former shareholders of ATBancorp as a group will receive shares in the Merger constituting approximately 25.2% of the outstanding shares of the combined company’s common stock immediately after the Merger. As a result, current shareholders of MidWestOne as a group will own approximately 74.8% of the outstanding shares of the combined company’s common stock immediately after the Merger.

Special Dividends (page 86)

The Merger Agreement provides that ATBancorp may pay a special dividend of $907.8452 per share to the ATBancorp shareholders at any time prior to the Closing as determined by ATBancorp.  Such dividend represents a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received as consideration in connection with the sale of United American Bank.  The special dividend was distributed to ATBancorp shareholders on September 24, 2018.

The Merger Agreement also provides that ATBancorp may pay a special dividend to shareholders based on the net proceeds (if any) received by ATBancorp in connection with the disposition by ATBancorp of the ATB Businesses prior to Closing, as required

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by the Merger Agreement. The payment of these special dividends by ATBancorp is in addition to the Merger consideration to be paid by MidWestOne. ATBancorp shareholders are not required to take any action to approve or receive these special dividends.

Recommendation of the MidWestOne Board of Directors (page 56)

After careful consideration, the MidWestOne board of directors recommends that MidWestOne shareholders vote “FOR” the MidWestOne merger proposal, “FOR” the MidWestOne stock issuance proposal and “FOR” the MidWestOne adjournment proposal (if necessary or appropriate).

For a more complete description of MidWestOne’s reasons for the Merger and the recommendations of the MidWestOne board of directors, please see the section entitled “The Merger—Recommendation of the MidWestOne Board of Directors and Reasons for the Merger” beginning on page 56.

Recommendation of the ATBancorp Board of Directors (page 66)

After careful consideration, the ATBancorp board of directors recommends that ATBancorp shareholders vote “FOR” the ATBancorp merger proposal and “FOR” the ATBancorp adjournment proposal (if necessary or appropriate).

In connection with entering into the Merger Agreement, MidWestOne entered into the Voting Agreement with certain holders of ATBancorp common stock. The shareholders that are party to the Voting Agreement beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock. The Voting Agreement requires that the shareholders party thereto vote all of their shares of ATBancorp common stock in favor of the Merger and against any proposal made in opposition to or in competition with the Merger. The Voting Agreement will terminate upon the earlier of the consummation of the Merger or the termination of the Merger Agreement in accordance with its terms. Pursuant to the Voting Agreement, approval of the ATBancorp merger proposal is virtually assured. For more information regarding the Voting Agreement, please see the section entitled “The Merger Agreement—Voting Agreement” beginning on page 99.

For a more complete description of ATBancorp’s reasons for the Merger and the recommendations of the ATBancorp board of directors, please see the section entitled “The Merger—Recommendation of the ATBancorp Board of Directors and Reasons for the Merger” beginning on page 66.

Opinions of Financial Advisors (pages 57 and 68)

Opinion of MidWestOne’s Financial Advisor

In connection with the Merger, MidWestOne’s financial advisor, Piper Jaffray & Co. (“Piper Jaffray”) delivered a written opinion, dated August 21, 2018, to MidWestOne’s board of directors as to the fairness, from a financial point of view and as of the date of the opinion, to MidWestOne of the aggregate Merger consideration in the proposed Merger. The full text of Piper Jaffray’s opinion, which describes the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by Piper Jaffray in preparing the opinion, is attached as Appendix C to this joint proxy statement/prospectus. This opinion was provided for the information and assistance of the MidWestOne board of directors in connection with its consideration of the Merger. The opinion did not address the merits of MidWestOne’s underlying decision to engage in the Merger or the relative merits of the Merger compared to any alternative business strategy or transaction in which MidWestOne might engage.  The opinion does not constitute a recommendation to the MidWestOne board of directors or any holder of MidWestOne common stock as to how any MidWestOne board member or such holder should vote with respect to the Merger.

For further information, please see the section entitled “The Merger—Opinion of MidWestOne’s Financial Advisor” beginning on page 57.

Opinion of ATBancorp’s Financial Advisor

In connection with the Merger, the ATBancorp board of directors received an opinion from Sandler O’Neill & Partners, L.P. (“Sandler O’Neill”), ATBancorp’s financial advisor. On August 20, 2018, Sandler O’Neill rendered its oral opinion to the ATBancorp board of directors (which was confirmed in writing by delivery of Sandler O’Neill’s written opinion dated August 20, 2018) to the effect that, as of such date, the per share Merger consideration to be received by holders of ATBancorp common stock in the Merger was fair to the holders of ATBancorp common stock, from a financial point of view.  The full text of Sandler O’Neill’s written opinion is attached as Appendix D to this joint proxy statement/prospectus. ATBancorp shareholders should read the entire opinion for a discussion of, among other things, the assumptions made, matters considered and qualifications and limitations on the review undertaken by Sandler O’Neill in rendering its opinion.

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Sandler O’Neill’s opinion speaks only as of the date of the opinion.  The opinion was directed to ATBancorp’s board of directors in connection with its consideration of the Merger Agreement and the Merger and does not constitute a recommendation to any shareholder of ATBancorp as to how any such shareholder should vote at any meeting of shareholders called to consider and vote upon the approval of the Merger Agreement and the Merger.  Sandler O’Neill’s opinion was directed only to the fairness, from a financial point of view, of the per share Merger consideration to the holders of ATBancorp common stock and does not address the underlying business decision of ATBancorp to engage in the Merger, the form or structure of the Merger or any other transactions contemplated in the Merger Agreement, the relative merits of the Merger as compared to any other alternative transactions or business strategies that might exist for ATBancorp or the effect of any other transaction in which ATBancorp might engage.

For further information, please see the section entitled “The Merger—Opinion of ATBancorp’s Financial Advisor” beginning on page 68.

MidWestOne Special Meeting of Shareholders (page 43)

The MidWestOne special meeting will be held at [·], at [·], Central Time, on [·]. At the MidWestOne special meeting, MidWestOne shareholders will be asked to approve the MidWestOne merger proposal, the MidWestOne stock issuance proposal and the MidWestOne adjournment proposal (if necessary or appropriate).

MidWestOne’s board of directors has fixed the close of business on [·] as the record date for determining the holders of MidWestOne common stock entitled to receive notice of and to vote at the MidWestOne special meeting. As of the MidWestOne record date, there were [·] shares of MidWestOne common stock outstanding and entitled to vote at the MidWestOne special meeting held by approximately [·] holders of record. Each share of MidWestOne common stock entitles the holder to one vote on each proposal to be considered at the MidWestOne special meeting. As of the MidWestOne record date, directors and executive officers of MidWestOne owned and were entitled to vote [·] shares of MidWestOne common stock, representing approximately [·]% of the shares of MidWestOne common stock outstanding on that date. MidWestOne currently expects that MidWestOne’s directors and executive officers will vote their shares in favor of the proposals to be presented at the special meeting, although none of them has entered into any agreements obligating them to do so. As of the MidWestOne record date, ATBancorp beneficially held no shares of MidWestOne common stock.

Approval of the MidWestOne merger proposal requires the affirmative vote of a majority of the votes entitled to be cast on the proposal.  Approval of the MidWestOne stock issuance proposal and the MidWestOne adjournment proposal requires the affirmative vote of a majority of the shares of MidWestOne common stock represented in person or by proxy at the MidWestOne special meeting and entitled to vote on such proposals.

ATBancorp Special Meeting of Shareholders (page 48)

The ATBancorp special meeting will be held at [·], at [·], Central Time, on [·]. At the ATBancorp special meeting, ATBancorp shareholders will be asked to approve the ATBancorp merger proposal and, if necessary or appropriate, the ATBancorp adjournment proposal.

ATBancorp’s board of directors has fixed the close of business on [·] as the record date for determining the holders of ATBancorp common stock entitled to receive notice of and to vote at the ATBancorp special meeting. As of the ATBancorp record date, there were 35,028 shares of ATBancorp common stock outstanding and entitled to vote at the ATBancorp special meeting held by [·] holders of record. Each share of ATBancorp common stock entitles the holder to one vote on each proposal to be considered at the ATBancorp special meeting. As of the ATBancorp record date, directors and executive officers of ATBancorp owned and were entitled to vote [·] shares of ATBancorp common stock, representing approximately [·]% of the shares of ATBancorp common stock outstanding on that date. In connection with entering into the Merger Agreement, MidWestOne entered into the Voting Agreement with certain holders of ATBancorp common stock. The shareholders that are party to the Voting Agreement beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock. The Voting Agreement requires that the shareholders party thereto vote all of their shares of ATBancorp common stock in favor of the Merger and against alternative acquisition proposals. The Voting Agreement will terminate upon the earlier of the consummation of the Merger or the termination of the Merger Agreement in accordance with its terms. As of the ATBancorp record date, MidWestOne beneficially held no shares of ATBancorp’s common stock.

Approval of the ATBancorp merger proposal requires the affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on this proposal. Pursuant to the Voting Agreement, approval of the ATBancorp merger proposal is virtually assured. Approval of the ATBancorp adjournment proposal

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requires the affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on this proposal.

ATBancorp’s Directors and Executive Officers Have Certain Interests in the Merger (page 80)

ATBancorp’s executive officers and directors have interests in the Merger that are different from, or in addition to, the interests of ATBancorp’s shareholders generally. Such interests include change in control agreements, deferred compensation agreements and salary continuation agreements with certain executive officers and the right to indemnification and insurance coverage following the consummation of the Merger. The members of the ATBancorp board of directors were aware of and considered these interests, among other matters, when they approved the Merger Agreement and recommended that ATBancorp shareholders approve the ATBancorp merger proposal. These interests are described in more detail under the section entitled “The Merger—Interests of ATBancorp Directors and Executive Officers in the Merger” beginning on page 80.

Management and Board of Directors of MidWestOne after the Merger (page 80)

Pursuant to the Merger Agreement, MidWestOne shall take all appropriate action, subject to and in accordance with MidWestOne’s articles of incorporation, bylaws, board policies and applicable legal requirements, to appoint two individuals to MidWestOne’s Board who are designated by ATBancorp, by a vote of at least 75% of ATBancorp’s Board, and are agreeable to MidWestOne. One of the individuals so designated by ATBancorp must not be a current officer, director or shareholder of ATBancorp and must have an established history in the Dubuque, Iowa business community (the “Dubuque Community Director”). One individual will be designated to serve as a Class I director and the other individual will be designated to serve as a Class II director, with the class assignments determined by MidWestOne, in each case, effective immediately upon the effectiveness of the Merger.

Regulatory Approvals Required for the Merger (page 84)

Completion of the Merger and the Bank Mergers are subject to various regulatory approvals, including approval or waiver from the Board of Governors of the Federal Reserve System (the “Federal Reserve Board”), and the approvals of the Federal Deposit Insurance Corporation (the “FDIC”), the Iowa Division of Banking (the “IDB”) and the Wisconsin Department of Financial Institutions (the “WDFI”).  The parties have filed notices and applications to obtain the necessary regulatory approvals or waivers, as applicable of the Federal Reserve Board, FDIC, the IDB and WDFI. On October 11, 2018, the Federal Reserve Board informed the parties that its legal division would not recommend that the Federal Reserve Board require the filing of an application by MidWestOne in connection with the Merger. The regulatory approvals to which completion of the Merger and Bank Mergers are subject are described in more detail under the section entitled “The Merger—Regulatory Approvals Required for the Merger” beginning on page 84.

Conditions to Consummation of the Merger (page 97)

The respective obligation of each party to effect the Merger is subject to the satisfaction or written waiver at or prior to the Effective Time of each of the following conditions:

·                  all actions and approvals by governmental authorities and certain other specified actions and approvals shall have been taken and obtained and shall remain in full force and effect and all statutory waiting periods shall have expired or been terminated and no such requisite regulatory approval shall have imposed a restriction or condition on, or requirement of, such approval that would, after the Effective Time, reasonably be expected by the board of directors of MidWestOne to materially restrict or burden, or impair in any material respect the benefits of the contemplated transactions to, or require a materially burdensome modification of, the businesses, activities, governance, legal structure, capital structure, compensation or fee arrangements of MidWestOne or its subsidiaries;

·                  ATBancorp and MidWestOne shall have each obtained the approval of the Merger Agreement and the transactions contemplated thereby by their respective shareholders;

·                  no order issued by any court or governmental agency or other legal restraint or prohibition preventing the consummation of the Merger or the other transactions contemplated by the Merger Agreement shall be in effect, and no legal requirement shall have been enacted, entered, promulgated or enforced by any governmental authority which prohibits or makes illegal consummation of the Merger;

·                  the S-4 registration statement of which this joint proxy statement/prospectus is a part shall have become effective under the Securities Act, and no stop orders suspending such effectiveness shall be in effect, and no proceeding shall have been commenced or be pending or threatened for such purpose; and

·                  MidWestOne shall have filed with the Nasdaq Stock Market, LLC a notification form for the listing of all shares of MidWestOne common stock to be delivered in the Merger, and the Nasdaq Stock Market, LLC shall not have objected to the listing of such shares.

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ATBancorp’s obligation to effect the Merger is also subject to the fulfillment or waiver of the following conditions:

·                  the accuracy of the representations and warranties of MidWestOne set forth in the Merger Agreement as of the date of the Merger Agreement and as of the Closing Date as though made at and as of the Closing Date (unless any such representation or warranty is made only as of a specific date, in which case as of such specific date);

·                  the performance by MidWestOne in all material respects of all covenants and obligations to be performed or complied with by it under the terms of the Merger Agreements on or prior to the Closing Date;

·                  the receipt of a certificate, signed on behalf of MidWestOne by an executive officer, certifying that certain of the conditions to ATBancorp’s obligation to complete the Merger have been satisfied;

·                  receipt by ATBancorp of a written opinion of its tax accountants to the effect that the Merger will constitute a “reorganization” within the meaning of Section 368(a) of the Internal Revenue Code; and

·                  from the date of the Merger Agreement to closing, no material adverse effect shall have occurred with respect to MidWestOne or any of its subsidiaries.

MidWestOne’s obligation to effect the Merger is also subject to the satisfaction or waiver of the following conditions:

·                  the accuracy of the representations and warranties of ATBancorp set forth in the Merger Agreement as of the date of the Merger Agreement and as of the Closing Date as though made at and as of the Closing Date (unless any such representation or warranty is made only as of a specific date, in which case as of such specific date);

·                  the performance by ATBancorp in all material respects of all covenants and obligations to be performed or complied with by it under the terms of the Merger Agreements on or prior to the Closing Date, and ATBancorp’s obtainment of and provision to MidWestOne of copies of all ATBancorp required approvals;

·                  the receipt of a certificate, signed on behalf of ATBancorp by an executive officer certifying that certain of the conditions to MidWestOne’s obligation to complete the Merger have been satisfied;

·                  the receipt by MidWestOne of a written opinion of its counsel to the effect that the Merger will constitute a “reorganization” within the meaning of Section 368(a) of the Internal Revenue Code;

·                  from the date of the Merger Agreement to closing, no material adverse effect shall have occurred with respect to ATBancorp or any of its subsidiaries;

·                  ATBancorp shall have satisfied all of the conditions necessary to complete the redemption of all of ATBancorp’s subordinated debentures bearing a fixed annual rate per annum of 6.50% and maturing September 15, 2021, in the aggregate principal amount of $9,600,000;

·                  the receipt of duly executed copies of the Lock-Up Agreements, which have been delivered;

·                  the total number of outstanding shares of ATBancorp common stock that have duly exercised their appraisal rights shall not exceed 7.5% of the outstanding shares of ATBancorp common stock;

·                  ATBancorp shall have completed the disposition of an aircraft and the ATB Businesses; and

·                  ATBancorp shall have delivered to MidWestOne all other instruments and documents that MidWestOne may reasonably request to effectuate the transactions contemplated in the Merger Agreement.

Acquisition Proposals (page 96)

Under the terms of the Merger Agreement, ATBancorp has agreed that it shall not, and it shall cause its subsidiaries and its subsidiaries’ representatives not to, directly or indirectly:

·                  initiate, solicit, encourage or knowingly facilitate any inquiries or proposals with respect to, or engage in any discussions or negotiations concerning, or provide any information, including any confidential or nonpublic information, or data to, or have any discussions with, any person relating to, any proposal or offer that constitutes, or could reasonably be expected to lead to, an acquisition proposal; or

·                  approve, recommend or enter into, or propose to approve, recommend or enter into, any letter of intent or similar document, memorandum of understanding, agreement, commitment, or agreement in principle with respect to an acquisition proposal.

ATBancorp has agreed to promptly notify MidWestOne of any such unsolicited acquisition proposal or of any request for information relating to ATBancorp or any of its subsidiaries that is reasonably likely to lead to or that contemplates an acquisition proposal, including the identity of the proposed acquirer and the material terms and conditions of such proposal.

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Termination of the Merger Agreement (page 98)

MidWestOne and ATBancorp may mutually agree at any time to terminate the Merger Agreement without completing the Merger. The Merger Agreement may also be terminated by MidWestOne or ATBancorp if:

·                  the other party breaches or fails to perform any of its representations, warranties, covenants or agreements set forth in the Merger Agreement, which breach or failure to perform, either individually or together with other such breaches, in the aggregate, if occurring or continuing on the date on which the Closing would otherwise occur would result in the failure of any of the conditions to closing and such breach or failure to perform has not been or cannot be cured on or prior to the earlier of two business days prior to June 30, 2019 (the “Termination Date”) and 30 days following written notice to the party committing such breach, making such untrue representation and warranty or failing to perform;

·                  (i) any regulatory authority that must grant a requisite regulatory approval has denied approval of any of the contemplated transactions and such denial has become final and nonappealable; (ii) any application, filing or notice for a requisite regulatory approval has been withdrawn at the request or recommendation of the applicable regulatory authority; or (iii) if (A) ATBancorp does not obtain the approval of the Merger Agreement and the transactions contemplated thereby by its shareholders or (B) MidWestOne does not obtain the approval of the issuance of shares of common stock in connection with the Merger by its shareholders, each following the ATBancorp shareholders’ meeting or the MidWestOne shareholders’ meeting, respectively; provided, however, that the right to terminate shall not be available to a party whose failure to fulfill any of its obligations under the Merger Agreement has been the cause of or resulted in the occurrence of any event described in clause (iii) above and such failure constitutes a material breach of the Merger Agreement;

·                  the Effective Time has not occurred at or before the Termination Date; provided, however, that the right to terminate shall not be available to any party whose failure to fulfill any of its obligations under the Merger Agreement has been the cause of, or has resulted in, the failure of the Effective Time to occur on or before such date and such failure constitutes a material breach of the Merger Agreement; or

·                  any court of competent jurisdiction or other regulatory authority shall have issued a judgment, order, injunction, rule or decree or taken any other action restraining, enjoining or otherwise prohibiting any of the contemplated transactions and such judgment, order, injunction, rule, decree or other action shall have become final and nonappealable.

Further, MidWestOne may terminate the Merger Agreement if:

·                  ATBancorp makes a Company Adverse Recommendation (as defined in the Merger Agreement) or if ATBancorp materially breaches certain of its other obligations under the Merger Agreement relating to holding a shareholders’ meeting or acquisition proposals; and

·                  if certain events, conditions or circumstances require further investigation, remedial or cleanup action under certain environmental laws and such expenditures are reasonably expected to exceed $10,000,000.

Further, ATBancorp may terminate the Merger Agreement if:

·                  if both of the following conditions are satisfied on the Determination Date, such termination to be effective on the 10th day following the Determination Date: (i) the Final Acquiror Market Value is less than $26.60; and (ii) the number obtained by dividing the Final Acquiror Market Value by $33.25 shall be less than the Index Ratio multiplied by 0.80; subject, however, to MidWestOne’s exercise of its option to increase the per share stock consideration payable to ATBancorp shareholders, in which event no termination shall occur; or

·                  under certain circumstances it receives an acquisition proposal that would constitute a superior proposal.

For more information, please see the section entitled “The Merger Agreement—Termination of the Merger Agreement” beginning on page 98.

Termination Fees (page 98)

The Merger Agreement also provides that upon termination of the Merger Agreement by either MidWestOne under certain circumstances, including ATBancorp failing to convene a meeting of its shareholders to consider and approve the Merger Agreement and the transactions contemplated thereby, or by ATBancorp by accepting a superior proposal, ATBancorp will be obligated to pay MidWestOne a termination fee of $7.6 million, plus the aggregate amount of MidWestOne’s expenses incurred in connection with the Merger, provided that the aggregate amount of such expenses shall not exceed $1,000,000.

Voting Agreement (page 99)

In connection with entering into the Merger Agreement, MidWestOne entered into the Voting Agreement with certain holders of ATBancorp common stock. The shareholders that are party to the Voting Agreement beneficially own in the aggregate

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62.41% of the outstanding shares of ATBancorp common stock. The Voting Agreement requires that the shareholders party thereto vote all of their shares of ATBancorp common stock in favor of the Merger and against any proposal made in opposition to or in competition with the Merger. The Voting Agreement will terminate upon the earlier of the consummation of the Merger or the termination of the Merger Agreement in accordance with its terms. Pursuant to the Voting Agreement, approval of the ATBancorp merger proposal is virtually assured.  For more information regarding the Voting Agreement, please see the section entitled “The Merger Agreement—Voting Agreement” beginning on page 99.

Lock-Up Agreements (page 99)

In connection with entering into the Merger Agreement, MidWestOne entered into Lock-Up Agreements (“Lock-Up Agreements”) with certain holders of ATBancorp common stock. The shareholders that are party to the Lock-Up Agreements beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock. The Lock-Up Agreements require that the shareholders that are parties thereto, during the period commencing as of the date on which the Effective Time occurs and expiring 180 days thereafter, not directly or indirectly, with certain exceptions, take any action to: (i) offer, sell, contract to sell, sell any option, warrant or contract to purchase, purchase any option, warrant or contract to sell, transfer, pledge or otherwise dispose of, or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition or otherwise) of any shares of MidWestOne common stock whether owned as of the time of the Lock-Up Agreements or subsequently acquired, including shares of MidWestOne common stock received by the shareholder pursuant to the Merger Agreement; (ii) enter into any swap or other derivative transaction that transfers to another, in whole or in part, any of the economic benefits or risks of ownership of MidWestOne common stock, whether any such transaction is to be settled by delivery of MidWestOne common stock or other securities, in cash or otherwise; or (iii) publicly disclose an intention to effect any transaction contemplated by clause (i) or (ii).  If the Merger Agreement is terminated without the consummation of the Merger, the Lock-Up Agreements will automatically terminate. For more information regarding the Lock-Up Agreements, please see the section entitled “The Merger Agreement— Lock-Up Agreements” beginning on page 99.

Material U.S. Federal Income Tax Consequences of the Merger (page 100)

The Merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code. Assuming the Merger qualifies as a reorganization, a shareholder of ATBancorp generally will not recognize any gain or loss upon receipt of MidWestOne common stock in exchange for ATBancorp common stock in the Merger, and will recognize gain (but not loss) in an amount not to exceed any cash received as part of the Merger consideration (except with respect to any cash received in lieu of a fractional share of MidWestOne common stock, as discussed below under “Material U.S. Federal Income Tax Consequences of the Merger—Cash Received In Lieu of a Fractional Share of MidWestOne Common Stock”). It is a condition to the completion of the Merger that MidWestOne and ATBancorp receive written opinions from their tax counsel or tax accountants to the effect that the Merger will qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

Tax matters are complicated and the tax consequences of the Merger to each ATBancorp shareholder may depend on such shareholder’s particular facts and circumstances. ATBancorp shareholders are urged to consult their tax advisors to understand fully the tax consequences to them of the Merger. For more information, please see the section entitled “Material U.S. Federal Income Tax Consequences of the Merger” beginning on page 100.

Comparison of Shareholders’ Rights (page 104)

The rights of ATBancorp shareholders who continue as MidWestOne shareholders after the Merger will be governed by the articles of incorporation and bylaws of MidWestOne rather than by the articles of incorporation and bylaws of ATBancorp. For more information, please see the section entitled “Comparison of Shareholders’ Rights” beginning on page 104.

The Parties (pages 109 and 110)

MidWestOne Financial Group, Inc.

102 South Clinton Street

Iowa City, Iowa 52240

Phone: (319) 356-5800

MidWestOne is an Iowa corporation incorporated in 1983, is a bank holding company registered under the Bank Holding Company Act of 1956, as amended (the “BHC Act”) and a financial holding company under the Gramm-Leach-Bliley Act of 1999.  As of June 30, 2018, MidWestOne had consolidated total assets of $3.276 billion, total loans held for investment of $2.3 billion, deposits of $2.6 billion and shareholders’ equity of $346 million.

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ATBancorp

895 Main Street

Dubuque, Iowa 52001

Phone: (563) 582-1841

ATBancorp is an Iowa corporation incorporated in 1985 and a bank holding company registered under the BHC Act.  As of June 30, 2018, ATBancorp had consolidated total assets of $1.4 billion, total loans held for investment of $1.1 billion, deposits of $1.1 billion and shareholders’ equity of $146.0 million.

Risk Factors (page 36)

Before voting at the MidWestOne special meeting or ATBancorp special meeting, you should carefully consider all of the information contained in or incorporated by reference into this document, including the risk factors set forth in the section entitled “Risk Factors” beginning on page 36 or described in MidWestOne’s Annual Report on Form 10-K for the year ended on December 31, 2017 and other reports filed with the SEC, which are incorporated by reference into this document. Please see “Where You Can Find More Information” in the forepart of this document and “Incorporation of Certain Documents by Reference” beginning on page 148.

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CERTAIN FINANCIAL INFORMATION REGARDING MIDWESTONE AND ATBANCORP

Selected Financial Information of MidWestOne

The following table summarizes consolidated financial results achieved by MidWestOne for the periods and at the dates indicated and should be read in conjunction with MidWestOne’s consolidated financial statements and the notes to the consolidated financial statements contained in reports that MidWestOne has previously filed with the SEC. Historical financial information for MidWestOne can be found in its Annual Report on Form 10-K for the year ended December 31, 2017 and its Quarterly Report on Form 10-Q for the six-month period ended June 30, 2018. Please see the section entitled “Where You Can Find More Information” for instructions on how to obtain the information that has been incorporated by reference. You should not assume the results of operations for past years indicate results for any future period.

(a)         Return on average assets is determined by dividing net income by average assets.

(b)         Return on average equity is determined by dividing net income by average shareholders’ equity.

(c)          Net interest margin (FTE) is determined by dividing net interest income, reported on a fully tax-equivalent basis assuming a 35% tax rate, by average earning assets. As presented, net interest margin (FTE) is a non-GAAP financial measure, and the most directly comparable GAAP measure is net interest margin. MidWestOne believes that the presentation of net interest margin

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on a tax equivalent basis provides useful information to investors regarding its results of operations because it provides a more accurate representation of MidWestOne’s actual net interest margin. A reconciliation of net interest margin is presented below.

(d)         As presented, the adjusted efficiency ratio is a non-GAAP financial measure. MidWestOne believes that the presentation of the adjusted efficiency ratio provides useful information to investors regarding its results of operations because it provides additional insights as to MidWestOne’s current performance. A reconciliation is presented below.

The following tables provide a reconciliation of the non-GAAP measures to the most comparable GAAP equivalent, in each case as of the dates presented.

(1) Computed on a tax equivalent basis, assuming a federal income tax rate of 35%.

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(a)         Return on average assets is determined by dividing net income by average assets.

(b)         Return on average equity is determined by dividing net income by average shareholders’ equity.

(c)          Net interest margin (FTE) is determined by dividing net interest income, reported on a fully tax-equivalent basis assuming a 35% tax rate (for 2017) or a 21% tax rate (for 2018), by average earning assets. As presented, net interest margin (FTE) is a non-GAAP financial measure, and the most directly comparable GAAP measure is net interest margin. MidWestOne believes that the presentation of net interest margin on a tax equivalent basis provides useful information to investors regarding its results of operations because it provides a more accurate representation of MidWestOne’s actual net interest margin. A reconciliation of net interest margin is presented below.

(d)         As presented, the adjusted efficiency ratio is a non-GAAP financial measure. MidWestOne believes that the presentation of the adjusted efficiency ratio provides useful information to investors regarding its results of operations because it provides additional insights as to MidWestOne’s current performance. A reconciliation is presented below.

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The following tables provide a reconciliation of the non-GAAP measures to the most comparable GAAP equivalent, in each case as of the dates presented.

(1) Computed on a tax equivalent basis, assuming a federal income tax rate of 21% for 2018, and 35% for 2017.

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Unaudited Pro Forma Condensed Combined Financial Information

The following is the unaudited pro forma combined financial data for MidWestOne and ATBancorp, giving effect to the Merger. The unaudited pro forma combined balance sheet as of June 30, 2018 gives effect to the Merger as if it occurred on that date. The unaudited pro forma combined income statement for the year ended December 31, 2017 and for the six months ended June 30, 2018 gives effect to the Merger as if it occurred as of the beginning of each period.

The unaudited pro forma combined financial statements have been prepared using the acquisition method of accounting for business combinations under accounting principles generally accepted in the United States of America (“GAAP”). MidWestOne is the acquirer for accounting purposes. Certain immaterial reclassifications have been made to the historical financial statements of ATBancorp to conform to the presentation in MidWestOne’s financial statements.

A final determination of the fair values of ATBancorp’s assets and liabilities, which cannot be made prior to the completion of the Merger, will be based on the actual net tangible and intangible assets of ATBancorp that exist as of the date of completion of the transaction. Consequently, fair value adjustments to assets acquired (including identifiable intangible assets) and liabilities assumed could change significantly from those in the unaudited pro forma combined financial statements presented herein and could result in a material change in amortization of acquired intangible assets. In addition, the value of the final purchase price of the Merger will be based on the closing price of MidWestOne’s common stock immediately preceding the Closing Date. A closing price of MidWestOne common stock of $33.78 was used for purposes of presenting the unaudited pro forma combined balance sheet at June 30, 2018.

In connection with the plan to integrate the operations of MidWestOne and ATBancorp following the completion of the Merger, MidWestOne anticipates that it will incur nonrecurring charges, such as costs associated with systems implementation, severance and other costs related to exit or disposal activities. MidWestOne is not able to determine the timing, nature and amount of these charges as of the date of this joint proxy statement/prospectus. However, these charges will affect the results of operations of MidWestOne and ATBancorp, as well as those of the combined company following the completion of the Merger, in the period in which they are recorded. The unaudited pro forma combined income statements do not include the effects of the non-recurring costs associated with any restructuring or integration activities resulting from the Merger, as they are nonrecurring in nature and not factually supportable at this time.

The Merger Agreement provides that ATBancorp may pay a special dividend of $907.8452 per share to the ATBancorp shareholders at a time prior to the Closing determined by ATBancorp. Such dividend represents a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received by ATBancorp as consideration in connection with the sale, completed on May 4, 2018, of United American Bank. ATBancorp shareholders are not required to take any action to approve or receive this special dividend. The payment of this special dividend by ATBancorp is in addition to the Merger consideration to be paid by MidWestOne. The unaudited pro forma combined financial statements reflect the special dividend of $907.8452 per share that was distributed to ATBancorp shareholders on September 24, 2018. In addition, the unaudited pro forma combined financial statements reflect the removal of historical income and expense amounts recognized by ATBancorp from the operations of United American Bank.

The Merger Agreement also provides that, prior to Closing, ATBancorp dispose of an aircraft and the ATB Businesses; the assets and liabilities of the ATB Businesses consist primarily of customers, contracts and employees. ATBancorp may pay a special dividend to ATBancorp shareholders based on the net proceeds (if any) received by ATBancorp in connection with the disposition of the ATB Businesses, prior to Closing. ATBancorp shareholders are not required to take any action to approve or receive this special dividend. The payment of this special dividend by ATBancorp is in addition to the Merger consideration to be paid by MidWestOne. The unaudited pro forma combined financial statements reflect the removal of historical estimated amounts attributed to the ATB Businesses.The unaudited pro forma combined financial statements do not, however, include the special dividend to shareholders based on the net proceeds received in connection with the disposition of the ATB Businesses, as MidWestOne is not able to determine the amount of such dividend as of the date of this joint proxy statement/prospectus.