We report the electrical resistance and microstructure of sputter deposited copper thin films grown in an oxygen containing ion-beam sputtering atmosphere. For films thinner than 5 nm, 2-10% oxygen causes a decrease in film resistance, while for thicker films there is a monotonic increase in resistivity. X-ray reflectivity measurements show significantly smoother films for these oxygen flow rates. X-ray diffraction shows that the oxygen doping causes a refinement of the copper grain size and the formation of cuprous oxide. We suggest that the formation of cuprous oxide limits copper grain growth, which causes smoother interfaces, and thus reduces resistivity by increasing specular scattering of electrons at interfaces.