Although nickel-based catalysts display good catalytic capability and excellent corrosion resistance under alkaline electrolytes for water splitting, it is still imperative to enhance their activity for real device applications. Herein, we decorated Ni_0.85Se hollow nanospheres onto reduced graphene oxide (RGO) through a hydrothermal route, then annealed this composite at different temperatures (400 °C, NiSe₂-400 and 450 °C, NiSe₂-450) under argon atmosphere, yielding a kind of NiSe₂/RGO composite catalysts. Positron annihilation spectra revealed two types of vacancies formed in this composite catalyst. We found that the NiSe₂-400 catalyst with dual Ni-Se vacancies is able to catalyze the oxygen evolution reaction (OER) efficiently, needing a mere 241 mV overpotential at 10 mA·cm^-2. In addition, this catalyst exhibits outstanding stability. Computational studies show favorable energy barrier on NiSe₂-400, enabling moderate OH^- adsorption and O₂ desorption, which leads to the enhanced energetics for OER.