Display omitted •FeNiOOH nanosheets are grown via in-situ chemical oxidation of 3D FeNi foam (FNF).•Oxyhydroxide species can directly act as real active catalyst for water oxidation.•The FeNiOOH/FNF exhibits superior OER performance to commercial IrO2 catalyst.•The superb catalytic performance owes to rational composition and structure design. The design and synthesis of efficient and affordable electrocatalysts for water oxidation are essential to advance water splitting technology, which depends on developing earth-abundant catalytic materials with rational compositions and structures. Here, we propose a facile synthesis of FeNiOOH nanosheets grown on FeNi foam (FNF) by in-situ chemical oxidation as a novel 3D electrode for oxygen evolution reaction (OER). This FeNiOOH/FNF electrode shows outstanding electrocatalytic performance in alkaline electrolyte, including high OER activity that requires a small overpotential of 252 mV to achieve a current density of 10 mA cm−2, favourable OER kinetics with a low Tafel slope of 36.8 mV dec−1, and excellent operation stability for at least 50 h. The superior OER properties are resulted from the highly active FeNi-oxyhydroxide species and advantageous nanosheet array structure. This work presents an effective strategy to rationalize self-supporting electrodes consisting of nanostructured arrays on conductive substrates, creating new opportunities to search for more advanced materials towards a sustainable energy future.