A systematic study of the electronic structure of oxygen chemisorbed on Al(111) surface has been carried out using an iterative extended Hückel method. We have investigated several configurations of atomic and diatomic oxygen chemisorption models. In agreement with previous studies, we find that the angular resolved photoemission data are reasonably consistent with the atomic oxygen chemisorption model. However, we also note that a diatomic chemisorbed phase gives rise to similar spectra though the relative energy level spacing is not in very good agreement with experiments. It has recently been suggested that at low doses and at room temperature an overlayer and an underlayer may coexist. Our calculation indicates that a double layer oxygen chemisorption model is not necessarily inconsistent with the photoemission data. Furthermore, a small change in work function observed upon oxygen chemisorption is also consistent with the double layer chemisorption model. We also report result of electronic structure calculations on isolated atomic and diatomic layers using the extended tight binding method. These results are helpful in interpreting the complex electronic structure of the oxygen-aluminum system. © 1982.