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Mortgage Banking - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |
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Dec. 31, 2021 | Dec. 31, 2020
Mortgage Banking [Abstract] |  |
Annual servicing fee income rate | 0.28% |
Reserve for mortgage loan servicing putback expenses | $ 379 | $ 311
Liability to repurchase past due GNMA loans | $ 43,400 | $ 55,500

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- Definition Liability to Repurchase Nonconforming GNMA Loans + References No definition available. + Details
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Namespace Prefix: | obnk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Mortgage Loan Servicing Putback Expenses, Reserve + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Servicing Fee Income, Rate + References No definition available. + Details
Name: | obnk_ServicingFeeIncomeRate
Namespace Prefix: | obnk_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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