To investigate the summer ozone pollution, observationally constrained modelling was carried out to study atmospheric oxidation capacity (AOC), OH reactivity, OH chain length, and HO_x budget for three different ozone concentration levels in Shanghai, China. It shows that AOC, dominated by reactions involving OH radical during the daytime, has a positive correlation with ozone levels. Some key VOCs species are very important for the OH reactivity and also the ozone formation potential.