In aqueous media, the electrochemistry of Cu is complicated by surface layers of native oxides. It is difficult to experimentally control the growth of these oxide films. These oxides also penetrate into subsurface regions and interfere with other surface reactions. Here, we combine potential step experiments with numerical calculations to study such an interface in a neutral electrolyte of NaClO4. Both in situ and ex situ sources of surface oxidation are investigated. Electrodeposition of Cd2+ is studied in the presence of surface oxides on Cu. The deposition of Cd2+ and catalytic reduction of O-2 affect each other. This complicates direct coulometric measurement of the Cd-coverage. An alternative method is demonstrated to determine the coverage isotherm of electrodeposition under these conditions. A phenomenological framework is developed to characterize the electrochemical features of the unavoidable surface oxides of Cu.