We present detailed electronic structure, total- and orbital-density-of-states information for an ordered overlayer of oxygen on Al(100). Our calculations show that O(2p) levels are split upon chemisorption. We attribute the oxygen-related structure reported in photoemission experiments to O(2pσ) levels broadened by band-structure and crystalfield effects. The O(2pz) levels are predicted to disperse significantly due to hybridization with the substrate and may be observable in angularly resolved photoemission experiments. © 1977 The American Physical Society.