With improving PM_2.5 air quality, the tropospheric ozone (O₃) has become the top issue of China’s air pollution control. Here, we combine comprehensive observational data analysis with models to unveil the contributions of different processes and precursors to the change of O₃ during COVID-19 lockdown in the Yangtze River Delta (YRD), one of the most urbanized megacity regions of eastern China. Despite a 44 to 47% reduction in volatile organic compounds (VOCs) and nitrogen oxides (NO_x) emissions, maximum daily 8-h average (MDA8) ozone concentrations increase from 28 ppbv in pre-lockdown to 43 ppbv in lockdown period. We reproduce this transition with the WRF-Chem model, which shows that ~80% of the increase in MDA8 is due to meteorological factors (seasonal variation and radiation), and ~20% is due to emission reduction. We find that daytime photochemistry does not lead to an increase but rather a decrease of daytime O₃ production during the lockdown. However, the reduced O₃ production is overwhelmed by the weakened nitric oxide (NO) titration resulting in a net increase of O₃ concentration. Although the emission reduction increases O₃ concentration, it leads to a decrease in the O_x (O₃ + NO₂) concentration, suggesting reduced atmospheric oxidation capacity on a regional scale. The dominant effect of NO titration demonstrates the importance of prioritizing VOCs reduction, especially from solvent usage and the petrochemical industry with high emission ratios of VOCs/NO_x.