Document and Entity Information |
12 Months Ended |
|---|---|
Dec. 31, 2025 | |
| Document Information [Line Items] | |
| Document Type | 11-K |
| Entity Registrant Name | OLD SECOND BANCORP INC |
| Entity Central Index Key | 0000357173 |
| Amendment Flag | false |
| 401(k) Savings Plan and Trust | |
| Document Information [Line Items] | |
| Document Type | 11-K |
| Amendment Flag | false |