Boltzmann equation is numerically solved for the plasmas generated in the process of discharging of O2, O2/He and O2/Ar in the gas phase. Thereafter, electron energy distribution function, mean electron energy and power efficiency are obtained. The results indicate that electron energy distribution functions are typically non-Maxwellian. For the same reduced electric field (E/N), the mean electron energy for pure oxygen discharge is lower than O2/He and O2/Ar mixtures. For the same oxygen content, the electron energy distribution function curves versus E/N for O2/He and O2/Ar mixtures cross together. The O2/Ar mixture discharge plasma has higher mean electron energy at lower E/N, while the O2/He mixture has higher mean electron energy at higher E/N. The influence of NO additive gas on the breakdown electric field can be negligible. Therefore, the improvement of O2(a1Δ) yield in the presence of NO gas may be not due to the slight decrease of E/N. An addition of 15% (volume fraction) O2(a1Δ) into O2 causes an increment of the mean electron energy which is not significant. In order to obtain the maximum yield of O2(a1Δ), the optimum E/N value of pure oxygen discharge plasma is about 10 Td. And the optimum E/N value decreases with the oxygen content. The mean electron energy as a function of the discharge frequency has a long steady baseline before its decrease. The power efficiency for excitation of O2(a1Δ) as a function of discharge frequency has two type curves. It decreases all the while at 10 Td. However, it rises and then goes down when the E/N is 50 Td. The optimum discharge frequency is 10 GHz for the conditions of 300 K and 1333.22 Pa.