In this work, we report a one-pot strategy to obtain ionic liquid–capped amorphous FexNiy hydroxide nanoclusters (<2 nm) with exceptional population of oxygen vacancies, which exhibits superior activity in oxygen evolution reaction (OER). Display omitted In this work, a one-pot strategy is presented to directly synthesize amorphous FexNiy hydroxide nanoclusters (denoted as ANC-FexNiy, <2 nm) with oxygen vacancies induced by ionic liquids. The ANC-FexNiy catalyst presents abundant catalytic sites and high intrinsic conductivity. As such, the optimized ANC-Fe1Ni2 exhibits high activity in oxygen evolution reaction (OER) with a Tafel slope of 39 mV dec–1 and an overpotential of 266 mV at 10 mA cm−2. Notably, the optimized ANC-Fe1Ni2 shows an extraordinarily large mass activity of 3028 A gFeNi–1 at the overpotential of 300 mV, which is ∼24-fold of commercial RuO2 catalyst. The superior activity of these FexNiy hydroxide nanoclusters is ascribed to (i) the amorphous and distorted structure with abundant oxygen vacancies, and (ii) enhanced active site density by downsizing the ANC-FexNiy clusters. This strategy provides a novel route for enhancing OER electrocatalytic performance and highly encouraging for the future application of amorphous metal hydroxides in catalysis.