In biochemistry, an oxidase is an oxidoreductase (any enzyme that catalyzes a redox reaction) that uses dioxygen (O2) as the electron acceptor. In reactions involving donation of a hydrogen atom, oxygen is reduced to water (H2O) or hydrogen peroxide (H2O2). Some oxidation reactions, such as those involving monoamine oxidase or xanthine oxidase, typically do not involve free molecular oxygen.
The oxidases are a subclass of the oxidoreductases. The use of dioxygen is the only unifying feature; in the EC classification, these enzymes are scattered in many categories.