Atomic layer deposition (ALD) is a method to fabricate uniform and conformal thin films based on surface chemistry. Al2O3 (alumina) is the most widely and thoroughly studied material in the field of oxide ALD. Most previous mechanistic studies on ALD of Al2O3 have been focused on the surface reaction step of Al-containing precursor: for the popular trimethylaluminum, it is known that the surface becomes terminated with Al-CH3 groups. However, reaction mechanisms for oxygen sources other than H2O has been largely unexplored. In this study, the surface reaction mechanisms for the CH3-terminated alumina surface with H2O, H2O2, and O-3 oxidants were investigated at molecular level. Density functional theory (DFT) calculations were used to obtain the energy change along reaction coordinates. It is confirmed that all three oxidants considered in this study would show facile oxidation of surface CH3 into OH under ALD conditions. Still, the exact mechanisms and number of oxidant molecules required for a full reaction to generate hydroxyl-terminated surface vary. The molecular reactivity toward ALD of Al2O3 at low deposition temperatures for the three oxidizing agents are expected to be H2O < H2O2 < O-3.