The oxygen evolution reaction (OER) is the key anodic reaction in water electrolysis for hydrogen production. Herein, a novel two-dimensional (2D) CoS/Co-MOF composite electrocatalyst was successfully synthesized using a post-synthesis method. The as-synthesized 2D CoS/Co-MOF composite employed as an OER electrocatalyst exhibits an exceptionally low overpotential of 327 mV at a current density of 100 mA cm⁻², considerably outperforming most reported transition metal sulfide catalysts (e.g., NiS/MOFs: 368 mV, Co₉S₈@MoS₂/Co-MOFs: 350 mV). Furthermore, the resulting CoS/Co-MOF OER electrocatalyst demonstrates exceptional stability, maintaining stable catalytic activity after 20 h of constant-current operation and exhibiting minimal degradation after 30 days of air exposure. In addition, a systematic investigation of key parameters (e.g., the thioacetamide (TAA) content and reaction temperature) was conducted to identify the optimal process conditions. Moreover, the catalytic mechanism of CoS/Co-MOF electrocatalyst was further elucidated based on density functional theory (DFT) calculations. These results reveal that the introduction of CoS can modulate the d-band centre of the CoS/Co-MOF, thereby optimizing adsorption free energy and reducing the overpotential. This synergy between the structure and optimized synthesis parameters advances the sustainable development of high-performance OER electrocatalysts. This work offers a feasible method for designing efficient and durable electrocatalysts that can facilitate large-scale applications of water electrolysis for hydrogen production.