Company: OSBC
Filing Date: 2025-05-06
Form Type: S-4/A
Source: 0001104659-25-045103
Chunk: 1

Company: OLD SECOND BANCORP INC
Filing Date: 2025-05-06
Form: S-4/A
Chunk 1
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 Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐ 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.

| ​ | Large accelerated filer 
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 ​                       | ​ | ​ | Accelerated filer         
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| ​ | Non-accelerated filer   
 ☐                       
 ​                       | ​ | ​ | Smaller reporting company 
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| ​ | ​                       | ​ | ​ | Emerging Growth Company   
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If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction: Exchange Act Rule 13e-4(i) (Cross-Border Issuer Tender Offer) ☐ Exchange Act Rule 14d-1(d) (Cross-Border Third-Party Tender Offer) ☐ 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 Commission, acting pursuant to said Section 8(a), may determine.

TABLE OF CONTENTS Information contained herein is subject to completion or amendment. A registration statement relating to the shares of Old Second common stock to be issued in the merger has been filed with the U.S. Securities and Exchange Commission. These securities may not be sold nor may offers to buy be accepted prior to the time the registration statement becomes effective. This proxy statement/prospectus shall not constitute an offer to sell or the solicitation of an offer to buy nor shall there be any sale of these securities in any jurisdiction in which such offer, solicitation or sale is not permitted or would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction.

PRELIMINARY — DATED MAY 6, 2025 — SUBJECT TO COMPLETION

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MERGER PROPOSED — YOUR VOTE