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Loans - Major Classifications - Loan Concentrations (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Loans |  |
Growth in loan balance | $ 279.4 |
Total real estate |  |
Loans |  |
Loans receivable as a percentage of total portfolio | 77.90% | 78.00%

X
- Definition Represents the amount of increase in loan balance over previous year. + References No definition available. + Details
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Namespace Prefix: | osbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Represents the percentage of total portfolio that is represented by a particular financial receivable category. + References No definition available. + Details
Name: | osbc_LoansAndLeasesReceivableAsPercentageOfAggregatePortfolio
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=osbc_RealEstatePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |