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[Display omitted] • FeCoNiCrMo HEA plate was prepared for efficient OER in alkaline media. • High-valence Mo/Cr cause dynamic structural/compositional evolution during OER. • Mo is rapidly oxidized and dissolved, provoking the concomitant Cr leakage. • Absorbed/intercalated MoO 4 2- facilitates the generation of active β-NiOOH. • Cr dopants in active layer enhance OER by preventing the Ni/Co over-oxidation. We herein report free-standing FeCoNiCrMo high-entropy alloy (HEA) plate as an excellent oxygen evolution reaction (OER) catalyst with activity and durability superior to benchmarking NiFe layered double hydroxide (LDH). It requires overpotentials of 303 and 372 mV to reach current densities of 100 and 700 mA cm−2, respectively, and maintains its activity for at least 120 h in alkaline media. Its dynamic structural/compositional evolution is uncovered, in which Mo metals are rapidly oxidized and dissolved into electrolyte, provoking the concomitant Cr leakage and leaving behind reconstructed active NiFeCo oxyhydroxide nanostructures on the surface. Its enhanced OER (intrinsic) activity is found to be associated with the absorption of resultant Mo oxo-anion (MoO 4 2-) and the persistence of trace Cr atoms in surficial NiFeCo oxyhydroxide phase. This work offers useful insights into the origin of high OER activity of HEA catalysts and also provides a feasible way for mass-production of active OER catalysts. [ABSTRACT FROM AUTHOR]