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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

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