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v3.20.4

Cash and Due from Banks (Details) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Federal Reserve Bank Regulatory Reserve [Member] |  |
Other disclosures |  |
Balances | $ 0 | $ 12,400,000

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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_CompensatingBalancesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_CashAndCashEquivalentsAxis=osbc_FederalReserveBankRegulatoryReserveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |