A three-dimensional (3D) catalyst was fabricated by using N-doped graphene films as scaffolds and nickel nanoparticles as building blocks via a heterogeneous reaction process. This unique structure enables high catalyst loadings and optimal electrode contact, leading to a surprisingly high catalytic activity towards OER, which almost approaches that of the state-of-the-art precious OER electrocatalysts (IrO2). Moreover, the catalytic process features favourable electrode kinetics and strong durability during long-term cycling. The dual-active-site mechanism was proposed for this 3D catalyst, i.e., Ni/NiOOH and Ni-N(O)-C are both active sites. The enhanced performance is attributed to synergistic effects of N-doped graphene and Ni, which enhance the activities of both components for OER.