Loans - Troubled Debt Restructurings (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 24,105	$ 14,403	$ 33,535	$ 31,018
% of Loans	0.30%	0.19%	0.42%	0.40%
Term Extension \| Real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 7,217	$ 401	$ 14,695	$ 1,658
% of Loans	0.14%	0.01%	0.28%	0.03%
Term Extension \| Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 16,879	$ 14,002	$ 18,831	$ 29,360
% of Loans	0.79%	0.70%	0.88%	1.46%
Weighted average term extension (in months)	6 months 18 days	5 months 15 days	6 months 27 days	6 months 3 days
Term Extension \| Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 9		$ 9
% of Loans	0.05%		0.05%
Weighted average term extension (in months)	36 months		36 months
Term Extension \| Commercial real estate \| Real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 1,168	$ 141	$ 4,069	$ 861
% of Loans	0.04%	0.01%	0.15%	0.04%
Weighted average term extension (in months)	2 months	3 months 21 days	4 months 21 days	5 months 18 days
Term Extension \| Construction/land/land development \| Real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 4,733	$ 7	$ 8,829	$ 132
% of Loans	0.68%	0.00%	1.26%	0.02%
Weighted average term extension (in months)	6 months 3 days	6 months	6 months 27 days	6 months 6 days
Term Extension \| Residential real estate \| Real estate \| Single-family residential real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 1,316	$ 253	$ 1,797	$ 665
% of Loans	0.09%	0.02%	0.13%	0.05%
Weighted average term extension (in months)	9 months	16 months 15 days	10 months 9 days	24 months 3 days
Other-Than-Insignificant Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 416	$ 61	$ 7,388	$ 61
% of Loans	0.01%	0.00%	0.09%	0.00%
Other-Than-Insignificant Payment Delay \| Real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 366	$ 34	$ 7,307	$ 34
% of Loans	0.01%	0.00%	0.14%	0.00%
Other-Than-Insignificant Payment Delay \| Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 0	$ 27	$ 30	$ 27
% of Loans	0.00%	0.00%	0.00%	0.00%
Weighted average term extension (in months)		4 months	3 months	4 months
Other-Than-Insignificant Payment Delay \| Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 50		$ 51
% of Loans	0.27%		0.28%
Weighted average term extension (in months)	2 months		2 months
Other-Than-Insignificant Payment Delay \| Commercial real estate \| Real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 46	$ 0	$ 5,852	$ 0
% of Loans	0.00%	0.00%	0.22%	0.00%
Weighted average term extension (in months)	6 months		8 months 24 days
Other-Than-Insignificant Payment Delay \| Construction/land/land development \| Real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 42	$ 0	$ 42	$ 0
% of Loans	0.01%	0.00%	0.01%	0.00%
Weighted average term extension (in months)	3 months		6 months
Other-Than-Insignificant Payment Delay \| Residential real estate \| Real estate \| Single-family residential real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 278	$ 34	$ 1,413	$ 34
% of Loans	0.02%	0.00%	0.10%	0.00%
Weighted average term extension (in months)	3 months	4 months	3 months 18 days	4 months

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Period of increase in weighted-average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=obnk_RealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=obnk_ConstructionLandandLandDevelopmentLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=srt_SingleFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: