Company: OSBC
Filing Date: 2025-05-08
Form Type: 424B3
Source: 0001104659-25-046065
Chunk: 56

Company: OLD SECOND BANCORP INC
Filing Date: 2025-05-08
Form: 424B3
Chunk 56
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 Delaware law or Bancorp Financial’s bylaws), and to use the additional time to solicit additional proxies, including from stockholders who have previously voted against the merger proposal.

The Bancorp Financial board of directors unanimously recommends that Bancorp Financial stockholders vote “FOR” approval of the Bancorp Financial adjournment proposal.**

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TABLE OF CONTENTS

### THE MERGER
This section of the proxy statement/prospectus describes material aspects of the merger. This summary may not contain all of the information that is important to you. You should carefully read this entire proxy statement/prospectus and the other documents we refer you to for a more complete understanding of the merger. In addition, we incorporate important business and financial information about Old Second into this proxy statement/prospectus by reference. You may obtain the information incorporated by reference into this proxy statement/prospectus without charge by following the instructions in the section entitled “Where You Can Find More Information” in the forepart of this proxy statement/prospectus.

#### Terms of the Merger
Each of Old Second’s and Bancorp Financial’s respective board of directors has approved the merger agreement. The merger agreement provides that Bancorp Financial will merge with and into Old Second, with Old Second continuing as the surviving corporation. Immediately following the merger, Evergreen Bank Group will merge with and into Old Second National Bank, with Old Second National Bank as the surviving bank.

In the merger, each outstanding share of Bancorp Financial common stock (except for treasury stock or shares owned by Old Second and Bancorp Financial, in each case, other than shares held on behalf of third parties or as a result of debts previously contracted, and shares held by Bancorp Financial stockholders who properly exercise dissenters’ rights) will be converted into the right to receive 2.5814 shares of Old Second common stock and $15.93 in cash, without interest.

Old Second will not issue any fractional shares of Old Second common stock in the merger. Instead, a Bancorp Financial stockholder who otherwise would have received a fraction of a share of Old Second common stock will receive an amount in cash equal to such fraction multiplied by the average closing price of Old Second common stock as quoted on NASDAQ during the five full trading days ending on and including the fifth trading day prior to the date the merger is consummated.

Holders of Bancorp Financial common stock are being asked to adopt the merger agreement. See the section entitled “The Merger Agreement” beginning on page