Using quantum mechanics calculations, we have studied the segregation energy with adsorbed O and OH for 28 Pt3M alloys, where M is a transition metal. The calculations found surface segregation to become energetically unfavorable for Pt3Co and Pt3Ni, as well as for the most other Pt binary alloys, in the presence of adsorbed O and OH. However, Pt3Os and Pt3Ir remain surface segregated and show the best energy preference among the alloys studied for both adsorbed species on the surface. Binding energies of various oxygen reduction reaction (ORR) intermediates on the Pt(111) and Pt3Os(111) surfaces were calculated and analyzed. Energy barriers for different ORR steps were computed for Pt and Pt3Os catalysts, and the rate-determining steps (RDS) were identified. It turns out that the RDS barrier for the Pt3Os alloy catalyst is lower than the corresponding barrier for pure Pt. This result allows us to predict a better ORR performance of Pt3Os compared to that of pure Pt. (Chemical Equation Presented).