Developing cost-efficient electrocatalysts for oxygen evolution is vital for the viability of H-2 energy generated via electrolytic water. Engineering favorable defects on the electrocatalysts to provide accessible active sites can boost the sluggish reaction thermodynamics or kinetics. Herein, Co1-xS nanosheets were designed and grown on reduced graphene oxide (rGO) by controlling the successive two-step hydrothermal reaction. A belt-like cobalt-based precursor was first formed with the assistance of ammonia and rGO, which were then sulfurized into Co1-xS by L-cysteine at a higher hydrothermal temperature. Because of the non-stoichiometric defects and ultrathin sheet-like structure, additional cobalt vacancies (V'(Co)) were formed/exposed on the catalyst surface, which expedited the charge diffusion and increased the electroactive surface in contact with the electrolyte. The resulting Co1-xS/rGO hybrids exhibited an overpotential as low as 310 mV at 10 mA.cm(-2) in an alkaline electrolyte for the oxygen evolution reaction (OER). Density functional theory calculations indicated that the V'(Co) on the Co1-xS/rGO hybrid functioned as catalytic sites for enhanced OER. They also reduced the energy barrier for the transformation of intermediate oxygenated species, promoting the OER thermodynamics.