The behavior of oxygen adsorption on the surface of liquid Cu-Ag alloys was investigated by measuring their surface tension (σ) with the sessile drop method in the oxygen partial pressures (p_o2) between 2.5 × 10⁻¹¹ and 2.5 × 10⁻³ Pa. The oxygen adsorption (the surface excess concentration of oxygen) was calculated from the slope of dσ/d ln p_o2 by applying Gibbs adsorption isotherm, for liquid Cu, Cu-5 at%Ag, Cu-10 at%Ag, Cu-20 at%Ag and Ag. It was found that the oxygen adsorption increased with the oxygen partial pressure, up to saturation on the surface. The oxygen adsorption on the surface of liquid Cu-20 at%Ag alloys exhibited almost the same behavior as that of pure liquid Ag, because surface saturation was not achieved for the Cu-20 at%Ag alloys, even at high oxygen partial pressures. Thermodynamic calculations using Butler's model indicated that the mole fraction of Ag in the surface of liquid Cu-Ag alloys drastically increases to 0.81 when the mole fraction of Ag in the bulk is only 0.2. Thus, it is considered that the outermost surface of liquid Cu-20 at%Ag alloys contains an enhanced level of Ag, which determines the oxygen adsorption behavior on liquid Cu-20 at%Ag alloys.