FAIR VALUE MEASUREMENTS - Fair value of mortgages payable (Details)	6 Months Ended	12 Months Ended
Jun. 30, 2025 USD ($) Y	Dec. 31, 2024 USD ($) Y
FAIR VALUE MEASUREMENTS
Line of credit, fair value	$ 6,800,000
Mortgages payable
FAIR VALUE MEASUREMENTS
Fair value of mortgages payable	438,558,000	$ 398,934,000
Carrying value of mortgages payable, gross	452,781,000	424,978,000
Fair value less than carrying value	$ (14,223,000)	$ (26,044,000)
Weighted average interest rate	4.76%	4.56%
Mortgages payable \| Blended market interest rate
FAIR VALUE MEASUREMENTS
Long-term debt, measurement input	0.0582	0.0628
Mortgages payable \| Weighted average remaining term to maturity
FAIR VALUE MEASUREMENTS
Long-term debt, measurement input \| Y	6.3	6.1

Represents the amount of difference between the carrying value and the fair value.

No definition available.

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Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations before netting of deferred finance costs. Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

No definition available.

+ Details

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Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
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- Definition

Fair value portion of contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time.

+ References

+ Details

Name:	us-gaap_LinesOfCreditFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Value of input used to measure long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
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Balance Type:	na
Period Type:	instant

- Definition

Weighted average interest rate of long-term debt outstanding calculated over time.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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- Details

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Period Type:

- Details

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Period Type:

- Details

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