OLD SECOND BANCORP INC

Regulatory Filings • Oct 21, 2025

I

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): October 21, 2025

(Exact name of registrant as specified in its charter)

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

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:

​ ​	​	​
Title of each class	Trading Symbol(s)	Name of each exchange on which registered
Common Stock	OSBC	The Nasdaq Stock Market

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. ☐

I tem 8.01 Other Events

On October 21, 2025, the Registrant’s Board of Directors declared a cash dividend of $0.07 per share payable on November 10, 2025, to stockholders of record as of October 31, 2025.

2

SIGNATURES

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.

​	​	​
​	OLD SECOND BANCORP, INC.
​	​	​
​	​	​
Dated: October 21, 2025	By:	/s/ Bradley S. Adams
​	​	Bradley S. Adams
​	​	Executive Vice President, Chief Operating Officer and
​	​	Chief Financial Officer

3