This study examines the fundamental processes that shed lights on the water oxidation mechanisms in a series of calcium perovskite oxides, CaMO 3 . To elucidate competing reaction pathways, periodic DFT calculations are employed. In particular, we examine the competition between the two proton-electron transfer pathways for the production of hydrogen peroxide and the four protonelectron transfer pathways for OER. In the case of early transition metal (M) perovskites, oxygen species display π-bonding characteristics at the surface, which align with the conventional metaloxo intermediate-driven OER mechanism. However, Mn-and Fe-based perovskites have been observed to strength surface-adsorbed oxygen species, promoting hole localization between adjacent oxygen atoms instead of direct proton transfer. This results in a preference for hydrogen peroxide production over OER.