Energy-efficient water electrocatalysis requires an oxygen evolution reaction (OER) catalyst which is not only earth-abundant but also highly stable. Herein, a facile impregnation method is demonstrated to synthesize nickel oxide (NiO) nanoparticles (NPs) of two different size ranges, i.e., 3–7 nm and 15–30 nm, uniformly anchored on gC3N4 forming a nanocomposite. The NiO NPs/gC3N4 nanocomposites with varying NiO loadings are synthesized to find the optimum loading for the best OER activity. A mass loading of ∼ 30 % NiO is found to show the highest OER activity from the detailed electrochemical measurements. The best performing NiO NPs/gC3N4 electrocatalyst exhibits an overpotential of 360 mV to achieve current density of 10 mA cm−2 with Tafel slope of 65 mV/dec and TOR of 5.8 s−1 at 360 mV overpotential. The synergistic effect between carbon nitride sheet and NiO NPs leads to the superior OER performance and durability.