Experimental and theoretical studies were made to examine the effects of an excess O2 contaminant on the small signal gain of a gasdynamic laser driven by combustion. The experiment was carried out up to 20% O2 concentration in a mixture of CO2+N2+O2+H2O. The small signal gain decreased by 1.3% for every 1% increase in the O2 content. Theoretical analysis was made by using two different models of vibrational relaxation; one is the conventional 3-mode model which includes CO2(υ2), CO2(υ3), and N2 vibrational modes, and the other is a 4-mode model which contains an additional 02 vibrational mode. The latter model was found to agree well with the experimental results.