Regulatory & Capital Matters - Dividend Restrictions and Deferrals (Details)	3 Months Ended
Mar. 31, 2022
Dividend Restrictions and Deferrals
Number of previous years retained profit considered for dividend payment	2 years
Minimum capital requirements, minimum percentage required	2.50%

The minimum percentage of Tier 1 Common Capital required to be maintained in the next fiscal year following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

No definition available.

Name:	osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageNextTwelveMonths
Namespace Prefix:	osbc_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration