This study aims to elucidate the fundamental principles governing the competing four-electron O 2 evolution and two-electron H 2 O 2 evolution pathways, addressing critical challenges in electrocatalysis. We explore a series of perovskites, CaMO 3 , using periodic density functional theory (DFT) and thermodynamic stability analysis. We have particularly chosen this type of materials because of the tunable constitutions and structures with a strong octahedral tilting that exhibit superb OER reactivity comparable to that of precious metal oxides . Our simulations reveal that early transition metals preferentially stabilize metal-oxo complexes (O * ), supporting four concerted proton-electron transfer steps and favoring O 2 evolution from water. In contrast, transition metals such as Mn, Fe, and Co show a selectivity toward H 2 O 2 production. In light of these findings, we discuss the potential for H 2 O 2 production correlating with the presence of polarons in perovskite compounds and the stabilization of key reaction intermediates, O * . Furthermore, Pourbaix diagrams will be presented to demonstrate the pH-dependent thermodynamic stability of perovskites in aqueous environments.