Magnetic polarity stratigraphy and stable isotope data from Ocean Drilling Program Leg 202 Core 202-1237B-31H allow calibration of the late Oligocene climate transition to the geomagnetic polarity timescale (GTS 2004). Oxygen isotope (delta (super 18) O) values in the benthic foraminifer Cibicidoides mundulus decrease from approximately 2.20 ppm in upper Chron 9n (Oi2b) to approximately 1.90 ppm in upper Chron 8r. The mean decrease is smaller than suggested by a recent compilation of benthic foraminifer delta (super 18) O records. The decrease occurs in three steps from 323 to 321 meters composite depth (mcd) within Chron 8r (27.027-26.554 Ma), and is here termed the "Chron 8r delta (super 18) O shift." Linear interpolation within Chron 8r estimates an age of 26.35 Ma for the final decrease to lower values marking initiation of the late Oligocene climatic optimum. The initial decrease at approximately 26.7 Ma is closely associated with the last occurrence of Paragloborotalia opima (P21/P22 planktonic foraminifer zonal boundary) between 322.51 and 322.76 mcd in lowermost Chron 8r. This study is a modest step toward an integrated magnetic polarity and astronomical chronostratigraphy for the late Oligocene based on Site 1237.