The amount of decrease in income from entity's investee (such as unconsolidated subsidiaries and joint ventures).

No definition available.

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

- Definition

Amount paid to managing member or general partner for management of the day-to-day business functions.

+ References

No definition available.

+ Details

Name:	olp_PaymentForManagementFees
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents information pertaining to reimbursement of insurance cost.

+ References

No definition available.

+ Details

Name:	olp_RealEstateInsuranceReimbursement
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the aggregate costs related to management and acquisition of properties during the reporting period.

+ References

No definition available.

+ Details

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

- Definition

Represents the quarterly amount to be paid under the agreement with the related party for the current fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Represents management fees included in the quarterly amount to be paid under the agreement with the related party for the current fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Represents overhead expenses included in the quarterly amount to be paid under the agreement with the related party for the current fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Represents prepaid management fees included in the quarterly amount paid under the agreement with the related party for the current fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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=6415400&loc=d3e5070-113901

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 14
-Section F

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
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

Name:	us-gaap_PropertyManagementFeePercentFee
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

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_MajesticPropertyManagementCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_BoardOfDirectorsChairmanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_BoardOfDirectorsViceChairmanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_GouldInvestorsLPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_CoVenturerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_ExecutiveOfficersAndOthersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorPropertyClassAxis=olp_NetLeasePropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorPropertyClassAxis=olp_OperatingLeasePropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=olp_UnconsolidatedJointVenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions (Details) - USD ($)		3 Months Ended		6 Months Ended
Related Party Transactions (Details) - USD ($)	Jan. 01, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Related Party Transaction
Share based compensation charged to operations		$ 740,000	$ 585,000	$ 1,406,000	$ 1,162,000
Majestic
Related Party Transaction
Quarterly fees under compensation and services agreement		672,000	634,000	1,324,000	1,264,000
Overhead expenses under compensation and services agreement		49,000	49,000	98,000	98,000
Management fees under compensation and services agreement		261,000	223,000	503,000	446,000
Majestic \| Other assets
Related Party Transaction
Prepaid Management fees under compensation and services agreement				21,000
Chairman
Related Party Transaction
Quarterly fees under compensation and services agreement				65,625	65,625
Vice Chairman
Related Party Transaction
Quarterly fees under compensation and services agreement				26,250	26,250
Gould Investors L.P.
Related Party Transaction
Insurance Reimbursement		101,000	57,000	202,000	114,000
Joint venture partners
Related Party Transaction
Real estate management and acquisition costs		36,000	33,000	88,000	138,000
Executive officers and others
Related Party Transaction
Share based compensation charged to operations		382,000	312,000	726,000	622,000
Net lease tenants
Related Party Transaction
Property management fee (as a percent)	1.50%
Operating lease tenants
Related Party Transaction
Property management fee (as a percent)	2.00%
Unconsolidated JV \| Joint venture partners
Related Party Transaction
Management fees		37,000	326,000	72,000	326,000
Reduction of equity in earnings of unconsolidated joint ventures		$ 19,000	$ 163,000	$ 36,000	$ 163,000