OLD SECOND BANCORP INC 0000357173 false 0000357173 2026-07-22 2026-07-22

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United States

Securities And Exchange Commission
Washington, D.C. 20549

FORM 8-K

Current Report

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): July 22, 2026

(Exact name of registrant as specified in its charter)

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Delaware | 000-10537 | 36-3143493
(State or other jurisdiction of incorporation) | (Commission File Number) | (I.R.S. Employer Identification No.)

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37 South River Street
Aurora, Illinois 60507
(Address of principal executive offices) (Zip code)

(630) 892-0202
(Registrant’s telephone number, including area code)

N/A

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

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Title of each class | Trading Symbol(s) | Name of each exchange on which registered
Common Stock | OSBC | The Nasdaq Stock Market
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Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 under the Securities Act (17 CFR 230.405) or Rule 12b-2 under the Exchange Act (17 CFR 240.12b-2).

Emerging growth company ☐

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 13(a) of the Exchange Act. ☐

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Item 2.02 Results of Operations and Financial Condition

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On July 22, 2026, Old Second Bancorp, Inc. (the “Company’s”) issued a press release announcing its financial results for the second quarter ended June 30, 2026, along with certain other financial information. Copies of the Company’s press release and loan portfolio disclosures are attached as Exhibits 99.1 and 99.2, respectively.

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Item 9.01 Financial Statements and Exhibits

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Exhibit No. | ​ | Description
99.1 | ​ | Press Release of Old Second Bancorp, Inc. dated July 22, 2026
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99.2 | ​ | Loan Portfolio Disclosures for Old Second Bancorp, Inc. dated June 30, 2026
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104 | ​ | Cover Page Interactive Data File (the cover page XBRL tags are embedded within the Inline XBRL document)

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Signature

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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​ | OLD SECOND BANCORP, INC.
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Dated: July 22, 2026 | By: | /s/ Bradley S. Adams
​ | ​ | Bradley S. Adams
​ | ​ | Executive Vice President,
​ | ​ | Chief Operating Officer and
​ | ​ | Chief Financial Officer

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