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Fair Value Measurements (Details) - USD ($)	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Fair Value Measurements
Fair value transfer assets , level 1 to level 2	$ 0	$ 0

X

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

No definition available.

+ Details

Name:	osbc_FairValueAssetsLevel1ToLevel2TransfersAmount1
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_FairValueDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration