Company: OSBC
Filing Date: 2025-06-30
Form Type: 11-K
Source: 0000357173-25-000063
Chunk: 2

Company: OLD SECOND BANCORP INC
Filing Date: 2025-06-30
Form: 11-K
Chunk 2
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 included evaluating the accounting principles used and significant estimates made by management, as well as evaluating the overall presentation of the financial statements. We believe that our audits provide a reasonable basis for our opinion. Supplemental Information The supplemental information in the accompanying schedule of assets (held at end of year) as of December 31, 2024 has been subjected to audit procedures performed in conjunction with the audit of the Plan’s financial statements. The supplemental information is the responsibility of the Plan’s management. Our audit procedures included determining whether the supplemental information reconciles to the financial statements or the underlying accounting and other records, as applicable, and performing procedures to test the completeness and accuracy of the information presented in the supplemental information. In forming our opinion on the supplemental information, we evaluated whether the supplemental information, including its form and content, is presented in conformity with Department of Labor’s Rules and Regulations for Reporting under the Employee Retirement Income Security Act of 1974. In our opinion, the supplemental information is fairly stated, in all material respects, in relation to the financial statements as a whole. /s/ Plante & Moran, PLLC

| We have served as the Plan’s auditor since 2011. | ​ |   |
| ​                                                | ​ | ​ |
| Cleveland, Ohio                                  | ​ | ​ |
| June 30, 2025                                    | ​ | ​ |

4

Old Second Bancorp, Inc. Employees’ 401(k) Savings Plan and Trust Statements of Net Assets Available for Benefits

| ​                                               
 ​                                               
 ​                                               
 Assets                                          | ​ 
 ​ 
 ​ | ​            
 December 31, 
 2024         
 ​            |           ​ 
           ​ | ​ 
 ​ | ​    
 2023 
 ​    |          ​ 
          ​ | ​ 
 ​ 
 ​ |
|:------------------------------------------------|:--|:-------------|------------:|:--|:-----|-----------:|:--|
| Cash - noninterest bearing                      | ​ | $            |     290,582 | ​ | $    |    389,505 | ​ |
| Participant directed investments, at fair value | ​ | ​            |  99,305,158 | ​ | ​    | 87,147,377 | ​ |
| Receivables:                                    | ​ | ​            |           ​ | ​ | ​    |          ​ | ​ |
| Employer match contribution receivable          | ​ | ​            |      81,083 | ​ | ​    |