We examine oxygen incorporation in alpha-Al2O3 using electronic structure techniques. We demonstrate that the ground-state configuration is a peroxide split interstitial, which is more than 2 eV lower in energy than the oxide closed-shell interstitial species in alumina, which proves to be only a transition state. Our results have general implications for the nature of oxygen interstitials in close-packed oxides. (C) 2010 Elsevier B.V. All rights reserved.