Nanoengineering effective bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remains a major challenge in improving the performance of electrochemical energy conversion devices. In this study, we present a simple synthesis route for producing Co-Nx crystals that are effectively arranged on reduced graphene oxide layers (Co–N/RGO) through mechanochemical treatment. The Co–N/RGO catalyst performs well as a bifunctional oxygen electrocatalyst. Specifically, the Co–N/RGO-700 has an earlier ORR onset (0.91 V) and a half-wave potential (0.79 V) with a higher kinetic current density of 6.6 mA cm−2 than Pt/C, as well as a lower overpotential (430 mV) and Tafel slope (115 mV dec−1) for the OER. These results demonstrate outstanding performance compared to reported Co-based catalysts. Theoretical modeling and experimental results explore the Co–N/RGO active sites, as well as the vital role of electronic structure, abundant N content, and surface defects, confirming it as a potential electrode material for energy conversion applications.