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Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Consolidated Statements of Changes in Stockholders' Equity
Dividend declared and paid (per share)	$ 0.04	$ 0.04

X

- Definition

Presents the amount of dividend declared and paid per share, by the entity.

+ References

No definition available.

+ Details

Name:	osbc_DividendsDeclaredAndPaidPerShare
Namespace Prefix:	osbc_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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