The NO oxidation performance in a non-thermal plasma (NTP) reactor under realistic synthetic exhaust gas compositions is investigated. The gas compositions differ mainly in the NO-NO2 ratio and represent different modes of operation of a marine diesel engine. It is found that the maximum NO oxidation efficiency is independent on the NO-NO2 ratio. Up to 55 % of the NO is mainly oxidised to NO2 in all gas mixtures being analysed. However, the specific energy density needed to reach the highest NO oxidation varies with the gas composition between 15 and 60 J/L. The performance of the NTP-reactor was significantly improved by the addition of propene (C3H6) acting as an additional oxidising agent. The energy consumption for NO-NO2 conversion was found to be between 20 and 45 eV/NO, depending on the ratio of the added propene as well as the initial concentrations of nitrogen oxides.