The polymer electrolyte membrane (PEM) electrolyzers are burdened with costly iridium (Ir)-based catalysts and high operation overpotentials for the oxygen evolution reaction (OER). The development of earth-abundant, highly active, and durable electrocatalysts to replace Ir is a critical step in reducing the cost of green hydrogen production. Here we develop a Ru₅Mo₄O_x binary oxide catalyst that exhibits high activity and stability in acidic OER. The electron-withdrawing property of Mo enriches the electrophilic surface oxygen species, which promotes acidic OER to proceed via the adsorbate evolution pathway. As a result, we achieve a 189 mV overpotential at 10 mA·cm⁻² and a Tafel slope of 48.8 mV·dec⁻¹. Our catalyst demonstrates a substantial 18-fold increase in intrinsic activity, as evaluated by turnover frequency, compared to commercially available RuO₂ and IrO₂ catalysts. Moreover, we report a stable OER operation at 10 mA·cm⁻² for 100 h with a low degradation rate of 2.05 mV·h⁻¹.