AbstractStructural flexibility can be a desirable trait of an operating catalyst because it adapts itself to a given catalytic process for enhanced activity. Here, amorphous cobalt hydroxide nanocages are demonstrated to be a promising electrocatalyst with an overpotential of 0.28 V at 10 mA cm⁻², far outperforming the crystalline counterparts and being in the top rank of the catalysts of their kind, under the condition of electrocatalytic oxygen evolution reaction. From the direct experimental in situ and ex situ results, this enhanced activity is attributed to its high structural flexibility in terms of 1) facile and holistic transformation into catalytic active phase; 2) hosting oxygen vacancies; and 3) structure self‐regulation in a real‐time process. Significantly, based on plausible catalytic mechanism and computational simulation results, it is disclosed how this structural flexibility facilitates the kinetics of oxygen evolution reaction. This work deepens the understanding of the structure–activity relationship of the Co‐based catalysts in electrochemical catalysis, and it inspires more applications that require flexible structures enabled by such amorphous nanomaterials.