Oxygen effects have long been ambiguous: exacerbating, being indifferent to, or ameliorating the net photoinactivation of Photosystem II (PS II). We scrutinized the time course of PS II photoinactivation (characterized by rate coefficient k_i) in the absence of repair, or when recovery (characterized by k_r) occurred simultaneously in CO₂ ± O₂. Oxygen exacerbated photoinactivation per se, but alleviated it by mediating the utilization of electrons. With repair permitted, the gradual net loss of functional PS II during illumination of leaves was better described phenomenologically by introducing τ, the time for an initial k_r to decrease by half. At 1500 μmol photons m⁻² s⁻¹, oxygen decreased the initial k_r but increased τ. Similarly, at even higher irradiance in air, there was a further decrease in the initial k_r and increase in τ. These observations are consistent with an empirical model that (1) oxygen increased k_i via oxidative stress but decreased it by mediating the utilization of electrons; and (2) reactive oxygen species stimulated the degradation of photodamaged D1 protein in PS II (characterized by k_d), but inhibited the de novo synthesis of D1 (characterized by k_s), and that the balance between these effects determines the net effect of O₂ on PS II functionality.