Mechanisms responsible for enhanced compression ignition when ozone (O3) is added into the intake charge were explored in a single-cylinder, optically accessible, research engine configured for low-load advanced compression ignition experiments. Intake O3 concentrations were varied up to 35 ppm, with the intake pressure fixed at 1.0 bar. Cycle resolved measurements of in-cylinder O3 decomposition for both motored and fired operation were performed using an O3 absorption diagnostic that used filtered 266 nm light from a continuous wave arc lamp. For motored operation, it was found that starting at intake valve closure (IVC), O3 gradually decomposed into molecular (O2) and atomic (O) oxygen. Near top dead center (TDC), there was an inflection point where rapid O3 decomposition occurred. The location of the inflection point advanced with increased intake temperature, decreased charge mass O2 content, or when fuel was added. When O3 decomposition measurements were compared to single-zone kinetic simulation results, the trends were found to be well captured but the absolute agreement between was fairly poor. For fired conditions, a strong absorption signal was observed shortly after the rapid O3 decomposition—well before low-temperature heat release (LTHR)—that was attributed to the presence of hydroxyl (OH). A small amount of heat release coincided with this peak that complementary single-zone kinetic modeling indicates is the result of exothermic conversion of OH and fuel into water. A second absorption peak was also observed that coincided with high-temperature heat release (HTHR). Ozone addition was found stabilize engine combustion for a wide range of early and late direct injection (DI) timings, with the most advanced combustion phasing occurring for early DI. For the present engine, a 35 ppm intake O3 concentration had roughly the same effect as a 45 K increase in intake temperature.