Cobalt oxides as efficient oxygen evolution reaction (OER) electrocatalysts have received much attention because of their rich reserves and cheap cost. There are two common cobalt oxides, Co3O4 (spinel phase, stable but poor intrinsic activity) and CoO (rocksalt phase, active but easily be oxidatized). Constructing Co3O4/CoO heterophase can inherit both characteristic features of each component and form a heterophase interface facilitating charge transfer, which is believed to be an effective strategy in designing excellent electrocatalysts. Herein, an atomic arrangement engineering strategy is applied to improve electrocatalytic activity of Co3O4 for the OER. With the presence of oxygen vacancies, cobalt atoms at tetrahedral sites in Co3O4 can more easily diffuse into interstitial octahedral sites to form CoO phase structure as revealed by periodic density functional theory computations. The Co3O4/CoO spinel/rocksalt heterophase can be in situ fabricated at the atomic scale in plane. The overpotential to reach 10 mA cm−2 of Co3O4/CoO is 1.532 V, which is 92 mV smaller than that of Co3O4. Theoretical calculations confirm that the excellent electrochemical activity is corresponding to a decline in average p‐state energy of adsorbed‐O on the Co3O4/CoO heterophase interface. The reaction Gibbs energy barrier has been significantly decreased with the construction of the heterophase interface. A Co3O4/CoO heterophase interface at the atomic scale is elaborately constructed by incorporating oxygen vacancies. Theoretical calculation proves that with the existence of oxygen vacancies, the energy barrier for Co atom diffusion from tetrahedron sites to neighboring interstitial octahedron sites is dramatically decreased indicating more favorable formation of CoO. With the formation of the Co3O4/CoO heterophase interface, oxygen evolution reaction activity is enhanced effectively.