In this study, the effect of oxygen (O-2) concentration in the range of 0.000 1% (= 1 ppm) to 20% on the decay of hydroxyl (OH) density after pulsed streamer discharge is examined in a humid nitrogen-oxygen mixture under atmospheric pressure. The decay of OH density was measured using laser-induced fluorescence and compared with one-dimensional postdischarge simulations. The simulation results are consistent with the measured decay of OH density for all O-2 concentrations. Essentially, the OH decay was double exponential, consisting of initial rapid decay fort < 0.1 ms and subsequent slow decay fort > 1 ms, wheretrepresents the postdischarge time. In the slow decay phase, O-2 change in the order of 10 ppm markedly affected the OH decay rate when O-2 < 100 ppm. The sensitive effect of the small amount of O(2)can be attributed to the reaction, H+O-2+M -> HO2+M<i, which converts H atoms to HO2, and the subsequent OH production via H+HO2 -> OH+OH. Other production and loss processes of OH in the postdischarge phase were also examined.