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Oxygen evolution reaction (OER) is the key reaction for water splitting, which is used for hydrogen production. Oxygen vacancy engineering is an effective method to tune the OER performance, but the direct relationship between the concentration of oxygen vacancy and OER activity is not well understood. Herein, a series of Ni<i>_y</i>Ce_{100−<i>y</i>}O<i>_x</i> with different concentration of oxygen vacancies were successfully synthesized. The larger concentration of oxygen vacancies in Ni₇₅Ce₂₅O<i>_x</i> and Ni₅₀Ce₅₀O<i>_x</i> result in their lower Tafel slopes, small mass-transfer resistance, and larger electrochemical surface areas of the catalysts, which account for the higher OER activities for these two catalysts. Moreover, with a fixed current density of 10 mA/cm², the potential remains stable at 1.57 V for more than 100 h, indicating the long-term stability of the Ni₇₅Ce₂₅O<i>_x</i> catalyst.