A new type of oxygen sensor for use on oceanic profiling CTDs has a response time that ranges from 0.4 to 0.7 s, depending on temperature. This oxygen (O2) sensor was calibrated in the field against O2 concentrations that were determined from Winkler titrations of Niskin-bottle water samples. Calibration coefficients were obtained from single casts and composite casts. When casts were calibrated separately, the mean error in predicting O2 concentrations was 1.22 ± 0.72 µM O2, n = 19 casts, and was not significantly different from the error between Niskin-bottle samples, which was 1.01 ± 2.33 µm O2, n = 33 triplicate or duplicate bottles. When casts wore combined into a composite cast for an entire cruise, however, the mean error in the calibrations increased to 5.03 ± 4.06 µm O2, n = 19 casts. These results also indicate that only three Niskin bottles on a cast are required to obtain an error of 1.0 µm in predicted oxygen. For routine use, we recommend calibrating each cast separately using either three or four measured O2 concentrations to obtain calibration coefficients.