Development of low cost electrocatalysts with outstanding catalytic activity and stability for oxygen evolution reaction (OER) in acid is a major challenge to produce hydrogen energy from water splitting. Herein, a novel metal-free electrocatalyst consisting of a oxygen-functionalized electrochemically exfoliated graphene (OEEG) nanosheets array is reported. Benefitting from a vertically aligned arrays structure and introducing oxygen functional groups, the metal-free OEEG nanosheets array exhibits superior electrocatalytic activity and stability toward OER with a low overpotential of 334 mV at 10 mA cm⁻² in acidic electrolyte. Such a high OER performance is thus far the best among all previously reported metal-free carbon-based materials, and even superior to commercial Ir/C catalysts (420 mV at 10 mA cm⁻²) in acid. Characterization results and electrochemical measurements identify the COOH species in the OEEG acting as active sites for acidic OER, which is further supported by atomic-scale scanning transmission electron microscopy imaging and electron energy-loss spectroscopy. Density functional theory calculations reveal that the reaction pathway of dual sites that is mixed by zigzag and armchair edges (COOH-zig-corner) is better than the pathway of single site.