In this paper, the physical adsorption characteristics of oxygen in coal pores were systematically investigated by the Grand Canonical Monte Carlo and the COMPASS force field. Firstly, coal pore structures of different sizes were constructed by graphite slit models and different groups. Secondly, the physisorption behavior of oxygen in graphite slit models of different sizes was simulated. Finally, the physisorption behavior of oxygen in graphite slit models at different pressures and temperatures was analyzed. The results showed that the physisorption density and excess physical adsorption of oxygen were divided into the rapidly decreasing stage (0.4-0.7 nm), the slowly decreasing stage (0.7-1.4 nm), and the stable stage (1.4 nm-5 nm) with the increase of coal pores, and the excess oxygen physisorption amount was more sensitive to the change of pressure. The O2 isosteric heat of physisorption decreased with increasing pore size of coal. Oxygen is more strongly adsorbed by hydroxyl and ether bonds than by methyl, carboxyl and carbonyl groups. Through this study, the mechanism of oxygen physical adsorption in coal pores and the characteristics influenced by temperature and pressure can be better understood.