A significant augmentation of the oxidation rate of carbon monoxide and unburned hydrocarbons has been previously demonstrated when the two processes of atmospheric pressure dielectric barrier discharge and Diesel oxidation catalysis were used simultaneously. In the case of carbon monoxide oxidation, the rate increase was attributed to a heterogeneous reaction of ozone with carbon monoxide. Ozone injection is investigated along with the direct application of the plasma to the gas upstream of a Diesel oxidation catalyst on a Diesel engine test bench. When used on the Diesel engine exhaust in steady state conditions, depending on the engine regime, the two processes had different effects on the gas composition. Both processes induced heating of the catalyst: while the plasma treatment of the entire gas flux heated the gas itself, and therefore also heated the catalyst, the injection of ozone heated the catalyst solely through the heat of reaction of ozone with the adsorbed species. Mechanisms for these effects are proposed and the performance of each approach is discussed in terms of energy cost and technical feasibility.