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The octahedral sites in Co3O4usually possess better oxygen evolution reaction (OER) activity, which have been identified as the active sites. Atomic cationic vacancy defects can elicit localized electron redistribution to regulate the electronic structure of the host materials. In this paper, Co3O4nanosheet arrays with cationic vacancies at octahedral sites (Vx-Co3O4) have been synthesized by a facile three-step reaction process. Density functional theory (DFT) computational results further indicate that the cationic vacancies can increase the number of electrons per Co atom and optimize the adsorption energy of the intermediate product. As a result, the optimized V0.3-Co3O4/CC electrode with abundant cationic vacancies displays excellent OER performance in alkaline electrolytes with an overpotential of 240 mV at 10 mA cm–2, Tafel slope of 72.2 mV dec–1, and superior stability, which are much better than those of the pure Co3O4and commercial IrO2.