Abstract. An observation-based model coupled to the Master Chemical Mechanism (V3.3.1) and constrained by a full suite of observations was developed to study atmospheric oxidation capacity (AOC), OH reactivity, OH chain length and HOx (=OH+HO2) budget for three different ozone (O3) concentration levels in Shanghai, China. Five months of observations from 1 May to 30 September 2018 showed that the air quality level is lightly polluted or worse (Ambient Air Quality Index, AQI, of > 100) for 12 d, of which ozone is the primary pollutant for 10 d, indicating ozone pollution was the main air quality challenge in Shanghai during summer of 2018. The levels of ozone and its precursors, as well as meteorological parameters, revealed the significant differences among different ozone levels, indicating that the high level of precursors is the precondition of ozone pollution, and strong radiation is an essential driving force. By increasing the input JNO2 value by 40 %, the simulated O3 level increased by 30 %–40 % correspondingly under the same level of precursors. The simulation results show that AOC, dominated by reactions involving OH radicals during the daytime, has a positive correlation with ozone levels. The reactions with non-methane volatile organic compounds (NMVOCs; 30 %–36 %), carbon monoxide (CO; 26 %–31 %) and nitrogen dioxide (NO2; 21 %–29 %) dominated the OH reactivity under different ozone levels in Shanghai. Among the NMVOCs, alkenes and oxygenated VOCs (OVOCs) played a key role in OH reactivity, defined as the inverse of the OH lifetime. A longer OH chain length was found in clean conditions primarily due to low NO2 in the atmosphere. The high level of radical precursors (e.g., O3, HONO and OVOCs) promotes the production and cycling of HOx, and the daytime HOx primary source shifted from HONO photolysis in the morning to O3 photolysis in the afternoon. For the sinks of radicals, the reaction with NO2 dominated radical termination during the morning rush hour, while the reactions of radical–radical also contributed to the sinks of HOx in the afternoon. Furthermore, the top four species contributing to ozone formation potential (OFP) were HCHO, toluene, ethylene and m/p-xylene. The concentration ratio (∼23 %) of these four species to total NMVOCs is not proportional to their contribution (∼55 %) to OFP, implying that controlling key VOC species emission is more effective than limiting the total concentration of VOC in preventing and controlling ozone pollution.