The oxygen evolution reaction (OER) is a key electrochemical reac-tion relevant to energy storage. Ni-containing bimetallic oxyhydrox-ides are among the most active OER catalysts in alkaline medium, but the mechanism of OER on pure Ni oxyhydroxide remains un-clear. 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 center dot 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 center dot. The Ni-OO-Ni species is a side-on super-oxide 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 center dot, 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.