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Regulatory & Capital Matters - Dividend Restrictions and Deferrals (Details)	12 Months Ended
	Dec. 31, 2020
Dividend Restrictions and Deferrals
Number of previous years retained profit considered for dividend payment	2 years
Minimum capital requirements, minimum percentage required	2.50%

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- References

No definition available.

+ Details

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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.

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No definition available.

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Name:	osbc_DividendRestrictionsNumberOfPreviousYearsRetainedProfit
Namespace Prefix:	osbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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- Definition

The minimum percentage of Tier 1 Common Capital required to be maintained in the next fiscal year following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

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No definition available.

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Name:	osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageNextTwelveMonths
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration