It is critical to develop efficient oxygen evolution reaction (OER) catalysts with high catalytic properties for overall water splitting. Electrocatalysts with enriched vacancies are crucial for enhancing the catalytic activity of OER through defect engineering. We demonstrated the dealloying method in a reducing alkaline solution using the CoAl alloy foil as a precursor to produce a new oxygen-vacancy-rich cobalt hydroxide (O-Co(OH)) hierarchical dendrite. The as-synthesised O-Co(OH) showed superior electrocatalytic activities toward OER when compared to pristine cobalt hydroxide (-Co(OH)), which had a low onset overpotential of only 242 mV and a small Tafel slope of 64.9 mV dec. Additionally, for the high surface area provided by the hierarchical dendrite, both -Co(OH) and O-Co(OH) showed a superior activity as compared to commercial catalysts. Furthermore, they retained good catalytic properties without remarkably decaying at an overpotential of 350 mV for 12 h. The as-made O-Co(OH) has prospective applications as an anode electrocatalyst in electrochemical water-splitting technologies with the advantages of superior OER performances, large surface area and ease of preparation.