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Other Real Estate Owned (Tables) | 12 Months Ended
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Dec. 31, 2024
Other Real Estate Owned |
Schedule of activity in the other real estate owned (OREO) portfolio, net of valuation reserve | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Twelve Months Ended
​ |  | December 31,
Other real estate owned |  | 2024 |  | 2023 | ​ | 2022
Balance at beginning of period | ​ | $ | 5,123 | ​ | $ | 1,561 | ​ | $ | 2,356
Property additions, net of participation sold | ​ | ​ | 21,083 | ​ | ​ | 5,580 | ​ | ​ | 87
Less: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Proceeds from property disposals, net of participation sold and gains/losses | ​ | ​ | 2,845 | ​ | ​ | 1,749 | ​ | ​ | 778
Period valuation write-down | ​ | ​ | 1,744 | ​ | ​ | 269 | ​ | ​ | 104
Balance at end of period | ​ | $ | 21,617 | ​ | $ | 5,123 | ​ | $ | 1,561
Schedule of activity in valuation allowance | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Twelve Months Ended
​ |  | December 31,
​ |  | 2024 |  | 2023 |  | 2022
Balance at beginning of period | ​ | $ | 118 | ​ | $ | 856 | ​ | $ | 1,179
Provision for unrealized losses | ​ | ​ | 1,744 | ​ | ​ | 269 | ​ | ​ | 104
Reductions taken on sales | ​ | ​ | - | ​ | ​ | (1,007) | ​ | ​ | (427)
Balance at end of period | ​ | $ | 1,862 | ​ | $ | 118 | ​ | $ | 856
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Schedule of expenses related to foreclosed assets, net of lease revenue | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Twelve Months Ended
​ |  | December 31,
​ |  | 2024 |  | 2023 | ​ | 2022
Gain on sales, net | ​ | $ | (390) | ​ | $ | (256) | ​ | $ | (163)
Provision for unrealized losses | ​ | ​ | 1,744 | ​ | ​ | 269 | ​ | ​ | 104
Operating expenses | ​ | ​ | 1,801 | ​ | ​ | 434 | ​ | ​ | 193
Less: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Lease revenue | ​ | ​ | 935 | ​ | ​ | 48 | ​ | ​ | 4
Net OREO expense | ​ | $ | 2,220 | ​ | $ | 399 | ​ | $ | 130
​

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- Definition Tabular disclosure of expenses related to foreclosed assets, net of lease revenue. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings. + References No definition available. + Details
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- Definition Tabular disclosure of the change in valuation allowance of other real estate on properties acquired through foreclosure. + References No definition available. + Details
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Namespace Prefix: | osbc_
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- Definition Tabular disclosure of the change in other real estate on properties acquired through foreclosure. + References No definition available. + Details
Name: | osbc_RealEstateAcquiredThroughForeclosureRollForwardTableTextBlock
Namespace Prefix: | osbc_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

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