To elucidate feasible routes of producing CO from CH3 and unravel the effect of adsorbed O on CHx transformation, the reac-tivity of CHx (x=1–3) with and without the assistance of adsorbed atomic O on Ni(111) was explored using density functional theory calculations. The adsorption energies of CHx (x=0–3) were found to be significantly reduced on an O-preadsorbed Ni(111) surface compared to a pure surface. Furthermore, O-assisted one-step dehydrogenation of CHx (x=1–3) features energy barriers and thus is diffcult to proceed. In terms of energy, the direct dissociation of CH3 is favorable, except for the last CH dehydrogenation, which is energy intensive. Interestingly, in O-assisted two-step CH transformation to CO via CHO intermediate, the barrier is dramatically lowered. The successive dehydrogenations of CHxO (x=1–3) were also found to be a route for CO formation. Finally, two possible pathways from CH3 to CO are proposed: (a) CH3→CH2→CH→CHO→CO;(b) CH3→CH3O→CH2O→CHO→CO.