To develop low-cost, earth-abundant NiFebased materials as highly efficient oxygen evolution reaction (OER) electrocatalysts and to probe new catalytic species are still great challenges to now. Here, an in situ formation of OER active NiFe2O4-NiOOH nanosheet arrays is demonstrated as a highly efficient OER electrocatalyst by the anodization of Fe2O3 domains anchored on Ni(OH)2 nanosheet arrays. The as-converted product can deliver the current density of 30 mA cm−2 with a small overpotential of 240 mV, and only requires an overpotential of 410 mV to achieve an amazing huge current density of 3000 mA cm−2. In situ potential-dependent Raman spectroscopy reveals that Ni(OH)2 in the composite is easier to be oxidized to NiOOH than pure Ni(OH)2, and the newly formed NiOOH reacts with the nearby Fe2O3 to produce hybrid NiFe2O4-NiOOH. It is found that the cooperative effect of the in situ formed NiFe2O4 and NiOOH as well as the hydrophilic and aerophobic electrode surface make main contribution to the outstanding OER activity of the catalyst. This work will bring new perspectives to the recognition of the origin of NiFe composite materials for OER and provide a mild method to synthesize amorphous spinel materials at room temperature.