Deep air-staged combustion is one of the important technologies to further reduce NOx emissions, and the mechanism of oxygen influence on NO heterogeneous reaction under deep air-staged conditions is still unclear. This study explores the influence of minor change in oxygen concentration on NO formation in the primary combustion zone under deep air-staged combustion by experimental and density functional theory (DFT). Experimental results show that there is an optimal oxygen concentration for NOx formation under the deep staged combustion. The theoretical calculation further verifies the experimental results, and reveals that char model with one hydroxyl group is more conducive to the N-2 desorption than the char model with two hydroxyl groups at the mechanism level. Oxygen changes the reaction pathway of char-NO, and the reduction of NO to N-2 is accompanied by the formation of CO or CO2. Moreover, the char and CO2 gasification reaction promotes CO formation under low oxygen condition under deep air staging. The kinetic analysis also reveals that there is an optimal concentration of oxygen in promoting C-NO heterogeneous reduction. The carbonaceous surface with one hydroxyl group enhances char reactivity and increases the desorption rate of N-2 compared to char surface without hydroxyl group. The char surface containing two hydroxyl groups promotes the retention of the pyridine nitrogen in the structurally stable aromatic ring, which greatly reduces the N-2 desorption rate and inhibits NO reduction. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.