Homogeneous charge compression ignition (HCCI) experiments were performed in a single-cylinder diesel engine fueled with n-heptane at constant equivalence ratio of 0.3, intake temperature of 300 K, and engine speed of 1500 rpm. The intake manifold was seeded by ozone produced by a dielectric barrier discharge reactor with a wire-cylinder configuration. Experimental results showed that low ozone concentrations, i.e., < 50 ppm, have an important impact on the phasing of the cool and main flame in the engine. To interpret the engine results, the modeling of constant volume combustion at 20 and 40 bars was performed using a detailed chemical kinetic reaction mechanism. The kinetic modeling indicated that the fuel starts to oxidize via reaction with O-atoms produced by ozone decomposition (n-C7H16 + O -> C7H15 + OH) and that the ozone-accelerated production of OH speed up both cool and main flame. The production of O-atoms by decomposition of ozone during the compression stroke could be a way to control, cycle to cycle, two stage ignition fuels of HCCI engine.