Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal-oxide photoanodes remains a significant experimental and theoretical challenge. Here we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly upon hematite. We observe that hematite is able to access a reaction mechanism third order in surface hole density, assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits a remarkably low activation energy (Ea ~ 60 meV). Density functional theory is employed to determine the energetics of charge accumulation and O-O bond formation on a model hematite 110 surface. The proposed mechanism shows parallels with the function of oxygen evolving complex of photosystem II, and provides new insights to the mechanism of heterogeneous water oxidation on a metal oxide surface.