This dataset contains benthic foraminiferal oxygen isotope (δ18O) measurements from Integrated Ocean Drilling Program (IODP) Site U1386 located on the southwestern Iberian margin. Sediment cores were recovered during IODP Expedition 339 aboard the R/V JOIDES Resolution in 2011. The analyzed interval spans approximately 668–1200 ka and was used to establish an oxygen isotope stratigraphy and age model for the Middle Pleistocene section of Site U1386.Sediment samples were collected at an average resolution of approximately 30 cm along the composite depth scale (c-mcd). Samples were wet sieved into >150 μm, 150–63 μm and 63–38 μm fractions, and the >150 μm fraction was used for stable isotope analyses. Oxygen isotope measurements were performed on benthic foraminiferal tests, primarily Planulina ariminensis and, where absent, Cibicidoides ungerianus. Prior to analysis, specimens were cleaned using repeated methanol-ultrasonication procedures to remove contaminants.Samples from the upper part of the record were analyzed at Vrije Universiteit Amsterdam and Utrecht University using carbonate preparation devices coupled to isotope ratio mass spectrometers, whereas samples from the lower interval were analyzed at the EPOC laboratory using a Thermo Scientific MAT 253 isotope ratio mass spectrometer coupled to a KIEL IV carbonate preparation device. The dataset provides δ18O values and corresponding depth information for stratigraphic correlation and studies of Quaternary paleoclimatic and paleoceanographic variability in the eastern North Atlantic.