The rational design principle of highly active catalysts for the oxygen evolution reaction (OER) is desired because of its versatility for energy-conversion applications. Postspinel-structured oxides, CaB₂O₄ (B = Cr³⁺, Mn³⁺, and Fe³⁺), have exhibited higher OER activities than nominally isoelectronic conventional counterparts of perovskite oxides LaBO₃ and spinel oxides ZnB₂O₄. Electrochemical impedance spectroscopy reveals that the higher OER activities for CaB₂O₄ series are attributed to the lower charge-transfer resistances. A density-functional-theory calculation proposes a novel mechanism associated with lattice oxygen pairing with adsorbed oxygen, demonstrating the lowest theoretical OER overpotential than other mechanisms examined in this study. This finding proposes a structure-driven design of electrocatalysts associated with a novel OER mechanism.