In the context of power-to-gas systems, including powerto-gas-oxyfuel, possible storage of a mixture of CO2 and O-2 requires density data information and evaluation of equations of state. Densities of three CO2-O-2 binary systems were measured using a vibrating tube densitometer, and the forced path mechanical calibration method in the gas, liquid, and supercritical regions between 276 and 416 K and at pressures up to 20 MPa (maximum expanded uncertainties U(rho) = 0.0005 MPa, U(T) = 0.3 K, and U(p) = 15 kg.m(-3)). The mole fractions of the prepared CO2-O-2 mixtures are 0.726:0.274, 0.517:0.483, and 0.872:0.128. The Peng-Robinson cubic equation of state (PR EoS) and the EoS-CG, based on GERG-2008 (and implemented in Refprop v10.0), were considered for the analysis of the data. Comparisons were carried out with literature data. It appears that the data are overall better predicted by the EoS-CG than the PR EoS.