The development of efficient electrocatalysts for the oxygen evolution reaction (OER) is critical for the generation of renewable energy carrier hydrogen from water splitting. Recently, metal oxides derived from metal nitrides, sulfides, chalcogenides, and phosphides have been demonstrated to exhibit superior catalytic activity as electrocatalysts for the OER but otherwise are difficult to gain access to by conventional metal oxidation methods. In this work, we demonstrate a novel in situ chemical etching process of Ni(OH)₂ nanosheets followed by a thermal ammonolysis process to fabricate iron–nickel nitride Ni₃FeN supported by pure Ni metal. Ni₃FeN is not stable and would be converted partially into NiFeOOH under oxygen-evolution conditions, forming a NiFeOOH/Ni₃FeN/Ni heterojunction showing extraordinarily high activity for the OER in alkaline medium with a low overpotential of 200 mV at 10 mA cm⁻² and a Tafel slope of 36 mV dec⁻¹. We demonstrated that NiFeOOH is the major active material while Ni₃FeN/Ni could help increase the charge transfer dramatically. More importantly, the presented NiFeOOH/Ni₃FeN/Ni catalyst has extremely good durability. This work provides a new strategy for the synthesis of metal and metal oxide heterojunctions for efficient electrocatalysts for the oxygen evolution reaction.