Oxygen vacancy engineering has been one of the most powerful strategies to enhance the efficiency of oxygen evolution reaction [OER]. Even though the positive effects of surface oxygen vacancy have been well proved, very controversial conclusions are also reported in perovskites. In this work, the roles of oxygen vacancies on the surface and in the bulk of La0.7Sr1.3Co2O6-δ (LSCO) are well separately investigated by X-ray absorption spectroscopy (surface sensitive) and magnetization measurement (bulk sensitive). The results suggest that massive surface oxygen vacancies have positive effects by enhancing the adsorption, while excessive oxygen vacancies in bulk is double-edged because of the interruptions of exchange interaction of cations with different valence states (Co–O–Co) and smooth transport of electrons. The sample prepared with massive surface oxygen vacancies while with less bulk oxygen vacancies yields 10 times higher specific activity at 1.6 V than commercial IrO2. This work uncovers the role of bulk oxygen vacancies in OER process and paves the way for smart oxygen vacancy engineering for higher OER efficiency.