Sputter-deposited Co/Cu multilayers at the 1st antiferromagnetic maximum (tCu= 9Å) exhibit large magnetoresistance and low relative hysteresis and are attractive for use in many position sensor applications. Unfortunately, their large magnetoresistance values can be hard to reproduce, perhaps a consequence of the difficulties in growing flat, pin-hole free layers. In this study we demonstrate that their magnetoresistive properties can be improved with the addition O2 to the Ar sputtering gas. A sequence of [Co(15Å)/Cu(9Å)]20 multilayers were made using magnetron sputtering in a 2mTorr Ar+O2 atmosphere, with the O2 concentration ranging from <1 ppm to 10,000 ppm. For films sputtered in our purest Ar, the magnetoresistance was typically less than 10%. Adding small amounts of O2 to the Ar improved the magnetoresistance, and at 1000 ppm O2 concentration it reached an optimal value of ∼45% and was found to be quite reproducible. The largest saturation fields were also achieved at 1000 ppm O2. Increasing the O2 concentration beyond 1000 ppm degraded the magnetoresistance and reduced the saturation fields. Magnetic measurements indicate that the improved magnetoresistance for films sputtered in Ar + O2 is primarily a consequence of more complete antiferromagnetic ordering at low fields. Auger depth profiling revealed no evidence for the O2 incorporation into the films, even for those fabricated in 10,000 ppm O2. Influence of the O2 on the microstructure of the multilayers is discussed.