The sluggish kinetics associated with the oxygen reduction reaction (ORR) at the proton exchange membrane fuel cell cathode leads to high overpotentials and limits fuel cell performance. Although significant progress has been made in first-principles modeling of the ORR, the complexity of the electrified aqueous/metal interface has limited advances in the use of theory to elucidate the influence of electrode potential on the mechanism and kinetics. The first reduction step of adsorbed molecular oxygen has been speculated to be the rate-determining step in the ORR. Periodic density functional theoretical calculations are carried out with the double-reference method developed by Filhol and Neurock [Angew. Chem. Int. Ed., 45, 402 (2006)] to determine the potential dependence of the reaction energy and activation barrier for the reduction of O2* to OO H* on the fully hydrated Pt(111) surface. This method allows for tuning the electrode potential with a slab representation of the electrode surface. Electron transfer is found to precede the protonation of the adsorbedO2 molecule, occurring with the proton formally residing as an H3 O+ species connected to the adsorbedO2 molecule by hydrogen bonding through two additional water molecules. The importance of the periodic representation of the metal electronic structure and the inclusion of extended solvation in considering the elementary kinetics is discussed.