Understanding the role of metal-oxygen covalency in the evolution of O 2 and its competitor H 2 O 2 is essential. Our DFT studies reveal that substituting transition metals at the M-site in the perovskite structure (CaMO 3 ) shifts the Fermi level across the metal d-band, which in turn alters the position of the d-band center relative to the O 2p-band center (Fig. ). As the d-band, from CaTiO 3 to CaNiO 3 , moves down in energy upon higher occupancy and approaches the O 2p states, the antibonding states below the Fermi level show increased oxygen character, indicating enhanced covalency in the metal-oxygen bond. When the O 2p states at the Fermi level are positioned above the d orbital and exceed the redox potential of the O 2 /H 2 O pair, as reported by Grimaud et al. , the oxidation of lattice oxygen in perovskites becomes thermodynamically favorable. Our calculations demonstrate that perovskites exhibiting the 3d 0 to 3d 2 configurations during the OER are associated with the weaker π-bond and linked to the conventional OER mechanism involving four-step proton-electron transfer processes (Fig. ). In contrast, 3d 3 to 3d 5 configurations involve the redox of the cations with σ character, deviating from the conventional OER mechanism and leading to the evolution of hydrogen peroxide (H 2 O 2 ) (Fig. ). This suggests that increased metal-oxygen covalency correlates with increasing H 2 O 2 evolution activity. According 5 to the Pauli principle, the participation of the e g orbitals affects the local structure around the transition metal cations, giving rise to the formation of polarons .