Together with hydrogen (H), oxygen is evolved by the electrolysis of water. The point of water electrolysis is to store energy in the form of hydrogen gas, a clean-burning fuel. The "oxygen evolution reaction (OER) is the major bottleneck [to water electrolysis] due to the sluggish kinetics of this four-electron transfer reaction."  All practical catalysts are heterogeneous.
Electrons (e) are transferred from the cathode to protons to form hydrogen gas. The half reaction, balanced with acid, is:
:2 H + 2e   →  H
At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and releasing electrons to the anode to complete the circuit:
:2 HO  →   O  +  4 H  +  4e
Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen:
:Overall reaction:
:2 HO   →   2 H  + O