The oxygen evolution reaction (OER) is a key electrochemical reaction relevant to energy storage. Ni-containing bimetallic oxyhydroxides are among the most active OER catalysts in alkaline medium, but the mechanism of OER on pure Ni oxyhydroxide remains unclear. Here we combine multiple operando spectroscopic tools including X-ray absorption, ultraviolet visible (UV-Vis), and Raman with electrokinetics to study the mechanism of OER on Ni(OH)₂ nanosheets. The spectroscopic data reveal two intermediates. The first one is a Ni(III)-O⋅ species formed upon a 2-e oxidation of Ni(OH)₂, and the second one is a Ni-OO-Ni species formed upon a 1-e oxidation of Ni(III)-O⋅. The Ni-OO-Ni species is a side-on superoxide that acts as a site for hole accumulation. The reaction kinetics follows an inner-sphere model. The rate-determining step is OH⁻ attack to Ni(III)-O⋅, chemically driven by the Ni-OO-Ni species. This work provides new experimental fingerprints and mechanistic perspectives for the understanding of Ni-based OER catalysts.