The pollution of atmospheric ozone in China shows an obvious upward trend in the past decade. However, the studies on the atmospheric oxidation capacity and O₃ formation in four seasons in the southeastern coastal region of China with the rapid urbanization remain limited. Here, a four-season field observation was carried out in a coastal city of southeast China, using an observation-based model combining with the Master Chemical Mechanism, to explore the atmospheric oxidation capacity (AOC), radical chemistry, O₃ formation pathways and sensitivity. The results showed that the average net O₃ production rate (14.55 ppb/hr) in summer was the strongest, but the average O₃ concentrations in autumn was higher. The AOC and ROx levels presented an obvious seasonal pattern with the maximum value in summer, while the OH reactivity in winter was the highest with an average value of 22.75 sec^-1. The OH reactivity was dominated by oxygenated VOCs (OVOCs) (30.6%-42.8%), CO (23.2%-26.8%), NO2 (13.6%-22.0%), and alkenes (8.4%-12.5%) in different seasons. HONO photolysis dominated OH primary source on daytime in winter, while in other seasons, HONO photolysis in the morning and ozone photolysis in the afternoon contributed mostly. Sensitivity analysis indicated that O₃ production was controlled by VOCs in spring, autumn and winter, but a VOC-limited and NOx-limited regime in summer, and alkene and aromatic species were the major controlling factors to O₃ formation. Overall, the study characterized the atmospheric oxidation capacity and elucidated the controlling factors for O₃ production in the coastal area with the rapid urbanization in China.