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Regulatory & Capital Matters - Dividend Restrictions and Deferrals (Details) | 9 Months Ended
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Sep. 30, 2019
Dividend Restrictions and Deferrals |
Number of previous years retained profit considered for dividend payment | 2 years

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- References No definition available. + Details
Name: | osbc_DividendRestrictionsAndDeferralsAbstract
Namespace Prefix: | osbc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the number of previous years retained profits combined with the current years profits used to determine the amount of dividends that may be paid in any calendar year. + References No definition available. + Details
Name: | osbc_DividendRestrictionsNumberOfPreviousYearsRetainedProfit
Namespace Prefix: | osbc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration