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Cash and Due from Banks (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Cash and Due from Banks.
Balances	$ 12.4	$ 10.8

X

- Definition

The amount of average compensating balance arrangement that is maintained with the Reserve Bank, under an agreement for a bank loan or future credit availability.

+ References

No definition available.

+ Details

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

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- References

No definition available.

+ Details

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