Based on DFT calculations, we propose a TM@CN hybrid structure, in which the single-atom transition metal (TM = Pt, Pd, Co, Ni, Cu) is supported by graphitic carbon nitride (g-CN), as a promising high-performance OER catalyst. Our work reveals the importance of local TM coordination in catalysts for the OER, which would lead to a new class of low-cost, durable and efficient OER catalysts.