A new mechanism of the oxygen evolving reaction catalyzed by [H2O(terpy)Mn(mu-O)(2)Mn(terpy)OH2](3+) is proposed by using density functional theory. This proton coupled electron transfer (PCET) model shows reasonable barriers. Because in experiments excess oxidants (OCl- or HSO5-) are required to evolve oxygen from water, we considered the Mn-2 complex neutralized by three counterions. Structure optimization made the coordinated OCl- withdraw a H+ from the water ligand and produces the reaction space for H2O2 formation with the deprotonated OH- ligand. The reaction barrier for the H2O2 formation from OH- and protonated OCl- depends significantly on the system charge and is 14.0 kcal/mol when the system is neutralized. The H2O2 decomposes to O-2 during two PCET processes to the Mn-2 complex, both with barriers lower than 12.0 kcal/mol. In both PCET processes the spin moment of transferred electrons prefers to be parallel to that of Mn 3d electrons because of the exchange interaction. This model thus explains how the triplet O-2 molecule is produced.