[en] The spatio-temporal variation of OH radical density in an atmospheric-pressure plasma discharge is investigated via two-dimensional numerical simulation. The behaviours of OH are characterized in four regions in the inter-electrode gap, namely the ‘Hot anode’, ‘Secondary streamer’, ‘Primary streamer’, and ‘Near-cathode’ regions. In these regions, temporal variations of averaged reduced electric field, electron density, and gas temperature differ and they affect the temporal variation of OH density. In all regions, a relatively large amount of OH is produced by the dissociation reactions of H₂O with electronically excited nitrogen molecules and oxygen atom rather than the direct electron-impact dissociation reactions. In the Hot anode and Secondary streamer regions, the instantaneous density of OH just after the discharge is high. However, the subsequent OH decay rates differ in these regions owing to the difference in gas temperatures. The reaction mechanism is discussed by dividing the effect of the diffusion-convection term and chemical reaction term in the continuous equation for OH radical density. (paper)