Electrocatalysis for the oxygen evolution reaction (OER) plays an irreplaceable role in numerous green and efficient energy conversion or storage techniques such as water electrolysis, fuel cells, and metal-air batteries. High-performance catalysts are always needed despite the sluggish kinetics of the four electron-transfer OER process. In this paper, for the first time, by employing a simple new strategy of "confined Fe overdoping", the OER activity of Ni3S2 in alkaline solution is significantly boosted, showing an overpotential of 350 mV at 10 mA cm-2, which is even lower than that of the benchmark IrO2. The designed catalyst (Meso C-NiFeS) is composed of mesoporous highly graphited N-doped carbon and nanodomain/defect/strain-rich NiFeS nanoparticles. The mesoporous carbon support facilitates mass/electron transfer, while confined Fe overdoping leads to smaller and defect/strain-rich nanodomains. DFT calculations prove that Fe doping could induce compressing strains, which is beneficial for the OER process, modify electronic states of Ni3S2, and act as active sites at the same time. This overdoping strategy can trigger a synergic effect combining size decrease, electronic structure modification, and defect/strain engineering. Moreover, this simple strategy is easy to implant to other catalysts.