RELATED PARTY TRANSACTIONS - Compensation and Services Agreement (Details) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Majestic
Related Party Transactions
Aggregate fees under compensation and services agreement	$ 785,000	$ 767,000
Property management costs allocated to real estate expenses	348,000	331,000
Additional payment for the entity's share of all direct office expenses	$ 74,000	$ 69,000
Majestic \| Net lease tenants
Related Party Transactions
Property management costs as a percentage of rental payments	1.50%	1.50%
Majestic \| Operating lease tenants
Related Party Transactions
Property management costs as a percentage of rental payments	2.00%	2.00%
Executive officers and others
Related Party Transactions
Stock incentive plan expense	$ 652,000	$ 449,000

Represents the amount of additional payment to related party for the share of direct office expenses.

No definition available.

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Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount to be paid under the agreement with the related party.

+ References

No definition available.

+ Details

Name:	olp_RelatedPartyTransactionFees
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of property management costs allocated to real estate expenses.

+ References

No definition available.

+ Details

Name:	olp_RelatedPartyTransactionPropertyManagementCostAllocatedToRealEstateExpenses
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

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

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num: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

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
Data Type:	na
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- Details

Name:	srt_MajorCustomersAxis=olp_OperatingLeasePropertyMember
Namespace Prefix:
Data Type:	na
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- Details

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