Development of high-efficiency single-atom catalysts (SACs) for the oxygen evolution reaction (OER) is challenging. Herein, we report a facile annealing strategy to construct an atomically dispersed cobalt- and nitrogen-codoped graphene catalyst for high-efficiency OER. The as-prepared 0.7-Co@NG-750 electrocatalyst requires an ultra-low onset overpotential of approximately 210 mV on a glassy carbon electrode in both 1.0 M KOH and 0.1 M KOH solutions. The density functional theory (DFT) calculations reveal that the theoretical overpotential on Co-N4 and Co-N2C2 sites is 0.41 and 0.53 V, respectively, and these might be efficient active sites for electrocatalytic water oxidation.