Acquisition - Bancorp Financial - Schedule of Purchase price of loans at acquisition (Details) - Bancorp Financial $ in Thousands	Jul. 01, 2025 USD ($)
Bancorp Financial Acquired PCD Loans
Par value of acquired loans	$ 89,870
Allowance for credit losses	(17,540)
Non-credit premium	722
Purchase price of PCD loans at acquisition	$ 73,052

Fair value of estimated contractual cash flow not expected to be collected for receivable purchased with credit deterioration, acquired in business combination.

No definition available.

Name:	osbc_BusinessCombinationAcquiredReceivablePurchasedWithCreditDeteriorationFairValue
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant