Laboratory studies of the effect of oxygen content in CO2 on the minimum miscibility pressure (MMP) are conducted on the n-C5H12/n-C16H34 model oil and Cottonwood Creek crude oil with three injection gases of different oxygen contents. The results indicate that the MMPs for these oils increase unfavorably with increasing O-2 concentration in the CO2 stream. The experimental results are also supported by our modeling work using a multiple-mixing-cell model, which is found to capture the effects of compositions and temperature, and is found to be a robust and predictive method for determining the MMP. Our experiments and calculations indicate that the effect of O-2 contamination on the MMP is larger for heavier oil and the effect of N-2 impurity on the MMP is larger than that of O-2 impurity.