Cobalt trioxide is a promising cobalt-based oxygen evolution reaction (OER) electrocatalyst, and its OER performance is limited by its low ratio of Co3+ to Co2+ ions and the dispersibility of nanoparticles. Here we report a simple method to improve the OER catalytic activity via loading Co3O4 nanoparticles onto boron carbon oxynitride (BCNO) nanosheets, which greatly decreases the overpotential (similar to 80 mV) and improves the stability of the Co3O4 nanoparticles. The Co3O4/BCNO nanocomposite exhibits a good OER performance with a small Tafel slope of 57.58 mV/dec and a low overpotential of 317 mV (vs RHE at 10 mA/cm(2)) under alkaline conditions. The high OER performance of the Co3O4/BCNO nanocomposite is induced by the larger active surface area and higher ratio of Co3+ to Co2+ ions as well as the formation of Co-N chemical bonds. (c) 2019 The Electrochemical Society.