The development of low cost and earth abundant efficient electrocatalysts is one of the effective strategies to enhance the oxygen evolution reaction (OER) activity. Herein, a facile strategy is demonstrated to synthesize OER active CoSn(OH)₆/g-C₃N₄ (CS/gCN) nanocomposite at room temperature. Among the prepared composites of different compositions, CS/gCN100 composite with ∼37 wt% gCN as confirmed by thermo gravimetric analysis exhibits a good OER activity with an overpotential of 336 mV at 10 mA cm⁻² and a tafel slope of 50 mV dec⁻¹ in 1 M KOH electrolyte, which is better than the commercial available standard IrO₂. The improved OER activity of CS/gCN100 of similar materials is ascribed to high specific surface area (107 m² g⁻¹), high electrochemical active surface area (4.55 cm²) and low charge transfer resistance (6.26 Ω) due to the synergistic effect of g-C₃N₄ and OER active CoSn(OH)₆. The OER activity of CS/gCN100 is further demonstrated by fabricating a two-electrode system with Pt/C as hydrogen evolution electrocatalyst for water splitting in an alkaline medium. The two-electrode system delivers an overall voltage of 1.66 V to achieve 10 mA cm⁻² with excellent durability for water splitting application.