The water-splitting reaction provides a promising mechanism to store renewable energies in the form of hydrogen fuel. The oxidation half-reaction, the oxygen evolution reaction (OER), is a complex four-electron process that constitutes an efficiency bottleneck in water splitting. Here we report a highly active OER catalyst, cobalt vanadium oxide. The catalyst is designed on the basis of a volcano plot of metal–OH bond strength and activity. The catalyst can be synthesized by a facile hydrothermal route. The most active pure-phase material (a-CoVOx) is X-ray amorphous and provides a 10 mA cm–2 current density at an overpotential of 347 mV in 1 M KOH electrolyte when immobilized on a flat substrate. The synthetic method can also be applied to coat a high-surface-area substrate such as nickel foam. On this three-dimensional substrate, the a-CoVOx catalyst is highly active, reaching 10 mA cm–2 at 254 mV overpotential, with a Tafel slope of only 35 mV dec–1. This work demonstrates a-CoVOx as a promising electrocatalyst for oxygen evolution and validates M–OH bond strength as a practical descriptor in OER catalysis.