Abstract The O2/CO/CO2 atmosphere and temperature dramatically affect NH3 reducing NO. Thermal DeNOx focuses on the effect of O2, while the temperature is limited below 1400 K. This paper investigated the effect of O2 on the NH3/NO reaction in a wider temperature range (1073–1773 K), especially emphasizing high temperatures above 1400 K, in four specific flow reactors. Additionally, three chemical kinetic models were used in simulations for verifying their applicability. It was proved that efficient NO reduction may be achieved in the temperature rising zone for the premixed reactants. Experimental results of gas-preheated reactors indicate that efficient NO reduction can be achieved beyond 1373 K in the absence of O2. While, in the presence of O2, the NH3/O2 reaction becomes more competitive with temperature increase in general. The SNCR temperature window was observed and PG2018 model made a pretty successful prediction of it. Besides, as temperature increases, the mixing mode begins to influence the final reaction results. In a special mixing mode, the NH3/NO reaction plays the dominant role above 1423 K, leading to the second temperature window. Moreover, near 1773 K, the outlet NO concentration tends to be equal to the initial value in almost all cases in gas-preheated reactors. Additionally, in NH3 oxidation experiments, little NO forms at high temperatures above 1673 K.