We present the ambient ozone and relevant observed trace gas dataset in Seoul, South Korea during the Megacity Air Pollution Studies (MAPS)-Seoul field campaign in May to June of 2015 (MAPS-Seoul 2015). We observed two distinctive periods, one with higher and the other with lower daytime ozone levels despite of the mostly clear conditions for the both periods. The importance of peroxy radical contributions to excess ozone production is illustrated by the substantial differences in the Leighton constant () for the two periods. Moreover, higher levels of hydroxyl radical (OH) reactivity (s–1) were observed during the high ozone episode compared to the low ozone episode as much as ~5 s–1. Contributions of nitrogen oxides (NOx) to OH reactivity become less important compared to volatile organic compounds (VOCs) during the high ozone episode, which suggests the NOx saturated ozone production regime. It was also notable that isoprene biogenic VOC contributes most to OH reactivity contribution calculated among from observed VOCs in the afternoon consistently over the whole field campaign period. Finally, we ran multiple box model scenarios to evaluate the ozone production rates of three different air mixtures: high ozone, low ozone, and a simulated regional air quality model mixture. The results illustrate that the total OH reactivity levels and the relative contributions of VOCs to NOx play critical roles in ozone production rates. The simulated regional air quality air mixture results in lower OH reactivity causing lower ozone production rates than those calculated for the high ozone episode air mixture. This exercise clearly urges further improvements in the regional model system to accurately simulate ozone precursors in the regions. This study clearly suggests that a comprehensive trace gas dataset along with observed OH reactivity allows us to properly diagnose the photochemistry behind ozone pollution, leading to effective ozone abatement policy implantations.