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Nitrogen dioxide and ozone are two interrelated oxidative pollutants in the atmosphere. Few studies have evaluated the health effects of combined oxidant capacity (O _x ). We investigated the short-term effects of O _x on fractional exhaled nitric oxide (FeNO), a well-established biomarker for airway inflammation, in a group of chronic obstructive pulmonary disease patients. Real-time concentrations of O _x were obtained by calculating directly the sum of nitrogen dioxide and ozone. Linear mixed-effect models were applied to explore the acute effects of O _x on FeNO levels. Short-term exposure to O_x was significantly associated with elevated FeNO. This effect was strongest in the first 24 h after exposure, and was robust to the adjustment of PM_2.5. A 10 μg m⁻³ increase in 24 h average concentrations of O _x was associated with 4.28% (95% confidence interval: 1.19%, 7.37%) increase in FeNO. The effect estimates were statistically significant only among males, elders, and those with body mass index ≥24 kg m⁻², a comorbidity, higher educational attainment, or moderate airflow limitation. This analysis demonstrated an independent effect of O _x on respiratory inflammation, and suggested that a single metric O _x might serve as a preferable indicator of atmospheric oxidative capacity in further air pollution epidemiological studies.