Mechanisms responsible for enhanced charge reactivity with intake added ozone (O3) were explored in a single-cylinder, optically accessible, research engine configured for low-load advanced compression ignition experiments. The influence of O3 concentration (0-40 ppm) on engine performance metrics was evaluated as a function of intake temperature and start of injection for the engine fueled by iso-octane, 1-hexene or a 5-component gasoline surrogate. For the engine fueled by either the gasoline surrogate or 1-hexene, 25 ppm of added O3 reduced the intake temperature required for stable combustion by 65 and 80 °C respectively.An ultraviolet (UV) light absorption diagnostic was also used to measure crank angle resolved in-cylinder O3 concentrations for select motored and fired operating conditions. The O3 measurements were compared to results from complementary 0D chemical kinetic simulations that utilized detailed chemistry mechanisms augmented with O3 oxidation chemistry. From the measurements, rapid thermally induced O3 decomposition was observed during the compression stroke shortly before top dead center. Ozone decomposition advanced when the charge temperature was increased, oxygen concentration was reduced, or fuel was added. While the model well-captures the experimental trends, for unfueled conditions the temporal prediction of O3 decomposition is generally too far retarded. The modeling further indicates the O3 decomposition leads to a burst of highly-reactive atomic oxygen (O). For fueled conditions, the O rapidly abstracts fuel hydrogen to form hydroxyl (OH), which then leads to substantial formation of hydroperoxyl (HO2) and hydrogen peroxide (H2O2). Strong UV light absorbance shortly after O3 decomposition confirms the presence of these species in the experiments. The in situ measurements are expected to aid kinetic model development of O3 decomposition processes and the associated influence of formed radicals on auto-ignition kinetics.