Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert O-2 in presence of hydrocarbons into CO or CO2 on demand. The underlying reaction mechanisms of plasma conversion of O-2 in the presence of hydrocarbons are analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of O-2 and of CH4. The plasma process is analysed with infrared absorption spectroscopy to monitor CH4 as well as the reaction products CO, CO2 and H2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from O-2 by electron. Oxygen atoms are efficiently converted into CO, CO2 and H2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced O-2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of O-2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion efficiency is enhanced by 15-20% already at temperatures of 430 K.