A grain boundary forms as an internal interface when two crystalline grains with mutually different crystallographic orientations are in direct contact with each other. As a result, atomic arrangement at grain boundaries differs from that of the bulk, showing serious displacements deviating from the original symmetric positions. As these symmetry-broken configurations are difficult to achieve in the bulk crystals, grain boundaries are considered distinctive platforms that can exhibit new physical properties. By using both sintered polycrystals with various grain sizes and thin films on bicrystal substrates, it is directly verified that surface-terminating grain boundaries in LaCoO3 and LaMnO3 are exceptional in oxygen evolution electrocatalysis, showing more than an order of magnitude higher activity. A combination of atomic-scale structure observation and density functional theory calculations demonstrates that the displacement of atoms in metal-oxygen octahedra correlates with significant splitting of the degenerate transition-metal 3d orbitals, and subsequently much easier charge transfer between metals and oxygen is attained. In addition to identifying the grain boundaries as strikingly active sites, the findings suggest that symmetry breaking by atom displacements in metal-oxygen octahedra is an efficient approach to remarkably enhance the oxygen electrocatalytic efficiency in perovskite oxides.