Water electrolysis has not yet been implemented on a large scale due to the sluggish oxygen evolution reaction (OER). Herein, we for the first time discover an interesting core-shell structure formation driven by the Kirkendall effect in a nanostructured FeNi alloy incorporating S, N-doped carbon (FeNi/SN-C) and this structural transformation can greatly boost the alloy's catalytic ability for OER. Thermal annealing of FeNi/SN-C in air induces the formation of an Fe-rich Fe-Ni oxide shell over the Fe-Ni alloy core due to the different metal diffusion rates and oxygen coupling abilities. As a powder catalyst, an overpotential as low as 230 mV can drive 10 mA cm-2, about 30 mV less than the original catalyst; it outperforms most nonprecious metal catalysts and noble commercial IrO2 catalysts. The catalytic performances are probably derived from the oxidized Fe-rich oxidation shell in contact with the conductive FeNi/SN-C host, which chemically stabilizes and further activates the active sites formed during the reaction. It is also concluded that exposure of the metal oxide shell contributes more to the activity than the large surface area contributed by the porous carbon matrix. This work puts forward a novel and efficient strategy to optimize Fe-Ni-based catalysts for OER by in situ structure and morphology tuning.