Developing highly active oxygen evolution reaction (OER) catalysts with fast OER kinetics is crucial for disruptively changing the energy technology, where unlocking of the catalytic origin is the key to the rational design of high-performance catalysts. Herein, a Co-based heterostructure consisting of cobalt (Co) and molybdenum carbide (Mo2C) nanoparticles in a 2D morphology is purposely designed as an OER precatalyst. At the initial stage of the OER in alkaline solution, the fast phase transition of Co metal into γ-phase cobalt oxyhydroxide (γ-CoOOH) in the presence of Mo2C gives rise to a Mo-enriched surface of the defective γ-CoOOH. This significantly raises the OER kinetics and gives an almost 90% enhancement in catalytic activity per metal site. Interestingly, the phase transition to γ-CoOOH and Mo-enriched surface reconstruction are potential-dependent and are accelerated at 1.4 V, as revealed by in situ Raman spectroscopy as well as ex situ scanning transmission electron microscopy studies. Potential-dependent X-ray photoelectron spectroscopy analyses and methanol oxidation experiments further confirm that the Mo enrichment into the defective CoOOH surface promotes electron flow from Mo to Co sites via the bridging oxygen, greatly benefiting the electrostatic adsorption of OH– ions and smoothing the subsequent OER steps.