Adsorption of oxygen on ultrathin Cu/Pt(111) films was studied using Auger electron spectroscopy, low-energy electron diffraction, and quadrupole mass spectroscopy. Comparing the oxygen-saturated copper films, the density of adsorbed oxygen is doubled as the copper coverage is reduced from 2 to 0.4 monolayers. In the submonolayer range, the more edge sites at the copper islands and larger Cu-Cu distances due to pseudomorphic growth provide a higher possibility for the adsorption of oxygen. A different adsorption rate of oxygen was resolved for the exposure of copper films to oxygen. As oxygen starts to incorporate into the film, the adsorption rate decreases. For a film prepared by alternate steps of copper deposition and oxygen adsorption, the oxygen distribution is not homogeneous from sputter profiling measurements. The amount of oxygen at both interfaces is higher than within the film. From the study of the annealing effects for Cu/Pt(111), the onset of diffusion for Cu adatoms into the Pt(111) substrate is slightly influenced by oxygen.