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Investment in Unconsolidated Joint Ventures (Details) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2015
item
property
Mar. 31, 2014
Dec. 31, 2014
item
property
Investment in Unconsolidated Joint Ventures      
Number of unconsolidated joint ventures 4olp_NumberOfUnconsolidatedJointVentures   5olp_NumberOfUnconsolidatedJointVentures
Number of properties owned and operated by each unconsolidated joint venture 1olp_NumberOfPropertiesOwnedAndOperatedByEachUnconsolidatedJointVenture   1olp_NumberOfPropertiesOwnedAndOperatedByEachUnconsolidatedJointVenture
Investment in unconsolidated joint ventures $ 2,486,000us-gaap_RealEstateInvestmentsUnconsolidatedRealEstateAndOtherJointVentures   $ 4,907,000us-gaap_RealEstateInvestmentsUnconsolidatedRealEstateAndOtherJointVentures
Equity in earnings of unconsolidated joint ventures 147,000us-gaap_IncomeLossFromEquityMethodInvestments 133,000us-gaap_IncomeLossFromEquityMethodInvestments  
Shopko retail stores, Lincoln, Nebraska      
Variable Interest Entities      
Ownership percentage in variable interest entity 50.00%us-gaap_VariableInterestEntityOwnershipPercentage
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= olp_UnconsolidatedJointVentureInLincolnNebraskaMember
   
Purchase of remaining investment in unconsolidated joint venture $ 6,300,000olp_PurchaseOfRemainingInvestmentInUnconsolidatedJointVenture
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= olp_UnconsolidatedJointVentureInLincolnNebraskaMember