In this work, a system of spinel FeCo2S4-catalyzed oxygen evolution reaction (OER) is studied. The hierarchical FeCo2S4 nanospheres (HNSs) are synthesized by a two-step hydrothermal method on Ni foam (NF) by vulcanizing Fe-Co precursors with different concentrations of Na2S. Since FeCo2S4 HNSs have the advantages of a large exposed surface area and high dispersity on the Ni foam, the FeCo2S4/NF heterostructure is used as a working electrode for OER in an electrochemical system. The catalysis results represent that the performance of FeCo2S4/NF in OER correlates with the vulcanized HNS surfaces made with different concentrations of Na2S, where the 0.1 M FeCo2S4/NF catalyst is the optimized condition to exhibit the lowest OER overpotential. Besides, the structures of FeCo2S4 HNSs are stable after a 12 h OER durability test. In the results of in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements with the FeCo2S4/NF electrode, the mechanism of FeCo2S4-catalyzed water splitting is through the formation of MOOH (M = metal) followed by the release of O2, the so-called indirect pathway, in the alkaline condition. Co has the major role to play in the OER reaction, while Fe holds its electronic state. Moreover, the reason for sulfide-assisted OER is the reduced charge-transfer barrier in the HNS structure that benefits MOOH generation.