We report a detailed first-principles analysis of the electronic structures of oxygen defective CeO₂ and Cu/CeO₂ surfaces aimed at elucidating the disappearance of the gap state of defective CeO₂ when a Cu atom is added at the surface. The top of the valence band of Cu/CeO₂ originates from the O 2p states around this Cu atom. We show that this redistribution of electronic states at the defective Cu/CeO₂ surface enhances the reactivity of the surface O atoms. Indeed, dynamical simulations show an acceleration of catalytic NO oxidation occurring via the Mars–van Krevelen mechanism mediated by these highly reactive oxygens.