The effects of oxygen on the interactions of the Cu-Al thin-film system are studied using both Cu/Al and Pt/Cu/Al structures. For the Cu/Al structure, oxygen-enhanced outdiffusion of Al and accumulation of Al on the Cu surface are observed. Using the Pt/Cu/Al structure, however, no surface accumulation of Al on Pt is observed; instead, the observed interdiffusion and alloy formation between Cu and Al are both suppressed by oxygen. At increasing temperatures, Al preferentially outdiffuses from the CuAl alloy to form a Pt Al alloy with the top Pt layer. This last process is also suppressed by oxygen, similar to the binary Pt/Al system reported previously. The mechanisms for the competing interactions and ambient effects observed are discussed.