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Basis of Preparation (Details 2) (USD $)	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012 property	Jun. 30, 2011 General and Administrative expenses	Jun. 30, 2011 General and Administrative expenses	Jun. 30, 2011 Real estate expenses	Jun. 30, 2011 Real estate expenses
Basis of Preparation
Number of properties sold, whose transferred assets and liabilities, are considered for reclassification	2
Number of properties deemed held for sale considered as held for sale	1
Prior period reclassification disclosures
Property management costs		$ 150,000	$ 300,000	$ (150,000)	$ (300,000)

X

- Definition

Represents the number of properties classified as properties deemed held-for-sale to be re-classified as properties held-for-sale as of end of last year period and at current balance sheet date.

+ References

No definition available.

+ Details

Name:	olp_NumberOfPropertiesAsDeemedHeldForSaleToBeReclassifiedAsPropertiesHeldForSale
Namespace Prefix:	olp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the number of properties classified as transferred assets and liabilites in prior year period and sold in the current period to be re-classified as properties held-for-sale as of end of last year period.

+ References

No definition available.

+ Details

Name:	olp_NumberOfPropertiesAsTransferredAssetsAndLiabilitesWhichSoldToBeReclassifiedAsPropertiesHeldForSale
Namespace Prefix:	olp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	olp_PriorPeriodReclassificationLineItems
Namespace Prefix:	olp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Definition

The amount of a reclassification adjustment made to prior period financial statement amounts.

+ References

No definition available.

+ Details

Name:	us-gaap_PriorPeriodReclassificationAdjustment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration