Using density functional theory (DFT) calculations with gradient-corrected exchange-correlation functionals (B3LYP), we systematically study the electronic structure and bonding of oxygen to Pt clusters. Our initial aim is to determine how the cluster size and shape affect the chemistry of dispersed catalysts and to find the most suitable cluster for modeling surface reactions on the Pt-surface. Consequently by including the effect of electric field and electrolyte through a polarizable continuum dielectric model, we determine the EDL effects on the surface chemistry and energetics of Oxygen interaction with catalyst surface. The results of this study yield useful insights into the precursors of oxygen reduction reaction (ORR) in the cathode of a PEM fuel cell