Density functional theory calculations were performed to investigate the pathways of the oxygen reduction reaction over the Pt-4 and Pt3M (M = Fe, V) clusters supported on the O-doped graphene substrate. The results show that the defect sites resulting from oxygen doping become the anchor sites for metal clusters. Our results also showed that O-2 adsorbs as a di-oxygen species on the supported Pt-4 and Pt3Fe clusters, but dissociates spontaneously on supported Pt3V. The di-oxygen species dissociates into co-adsorbed HO* and O* upon reduction on both supported Pt-4 and Pt3Fe and no stable HOO* intermediates were isolated. On supported Pt-4, further reduction via both HO* + HO* and H2O* + O* routes is possible, with reaction favoring the HO* + HO* route energetically. On supported Pt3Fe, the HO* + HO* and H2O* + O* routes are competitive. On supported Pt3V, the reduction reaction is likely to proceed exclusively through the HO* + HO* route as no stable co-adsorbed H2O and O* state was isolated. The results were discussed in the context of the experimentally observed enhancement of ORR reactivity on the graphene supported Pt3Cr and Pt3Co nanocatalysts. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.