Density functional theory (DFT) calculation with numerical atomic functions is used to study oxygen effects on MoS2 hydrotreating active sites. The optimal form of oxygen on active sites is first investigated by experimentation and calculation. Then, the fine structures of a MoS2 nanocluster (S-Mo-S) and an oxygen containing MoS2 nanocluster (O-Mo-S) under reaction conditions, as well as their hydrodesulfurization (HDS) selectivity towards thiophene, are calculated. The results show that bent MoS2 nanoclusters in supported hydrotreating catalysts are more thermodynamically stable when oxygen is substituted for sulfur on the basal plane. Oxygen also reduces the electron density and unoccupied d-orbital energy levels of Mb atoms. The S atoms on the Mo-edge are also more readily substituted by H on O-Mo-S. During thiophene HDS, O-Mo-S has a stronger thiophene adsorption ability than S-Mo-S. Hydrogen transfer has a higher energy requirement along the Mo-edge on O-Mo-S than on S-Mo-S. The hydrogenation saturation activity to C=C is lower on the Mo-edge of O-Mo-S than of S-Mo-S, whereas the hydrogenolysis activity at the C-S bond is promoted by oxygen. It is predicted that the oxygen can increase the hydrogenolysis selectivity of a MoS2 active nanocluster. (C) 2016 Elsevier Ltd. All rights reserved.