Molecular dynamics simulations are performed to investigate the physical adsorption of oxygen on graphite at 15 to 300 K with coverages from 0.192 to 2 monolayers. Below 25 K, the adsorbed molecules have translational ordering and are in a solid state. Above 25 K solid clusters at submonolayer coverages disintegrate to a liquid-like layer without translational ordering. For coverages close to or above monolayer, layering transition and some out-of-plane orientations are observed at high temperatures. More adsorbed molecules escape into the gas phase at higher temperatures and the equilibrium vapour pressure can be estimated by the bulk phase density. A phase diagram is constructed summarizing these changes and the adsorption isotherms.