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Efficient catalysts for the anodic oxygen evolution reaction (OER) are critical for electrochemical H₂ production. Their design requires structural knowledge of their catalytically active sites and state. Here, we track the atomic-scale structural evolution of well-defined CoO_x(OH)_y compounds into their catalytically active state during electrocatalytic operation through operando and surface-sensitive X-ray spectroscopy and surface voltammetry, supported by theoretical calculations. We find clear voltammetric evidence that electrochemically reducible near-surface Co³⁺–O sites play an organizing role for high OER activity. These sites invariably emerge independent of initial metal valency and coordination under catalytic OER conditions. Combining experiments and theory reveals the unified chemical structure motif as µ₂-OH-bridged Co^2+/3+ ion clusters formed on all three-dimensional cross-linked and layered CoO_x …