The sluggish oxygen reduction reaction (ORR) limits PEMFC device efficiency. Difficulties characterizing the adsorbed O2 state at the solvated, electrified interface lead to conflicting conclusions regarding the order of initial adsorption, electron transfer, proton transfer, and O-O dissociation steps. O2 adsorption strength and extent of charge transfer depend on the electrochemical environment and interfacial water structure. Despite extensive computational and experimental studies of O2 adsorption on the Pt(111) surface in UHV, solvation and potential effects on the O2-Pt(111) interaction are less well defined. We applied density functional theory methods to investigate O2 adsorption, within a bilayer water structure, under a homogeneous external electric field and under potential control using the double reference method. The O2 dissociation barrier and O2-water competitive adsorption are affected by the interfacial electric field and solvent. Constant electric field and constant electrode potential models give qualitatively different trends for O2 adsorption and dissociation.