The oxygen-evolving complex (OEC), also known as the water-splitting complex, is a water-oxidizing enzyme involved in the photo-oxidation of water during the light reactions of photosynthesis.^[2] OEC is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. The mechanism for splitting water involves absorption of three photons before the fourth provides sufficient energy for water oxidation.^[3] Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S₀ to S₄, with S₀ the most reduced and S₄ the most oxidized. Photons trapped by photosystem II move the system from state S₀ to S₄. S₄ is unstable and reacts with water producing free oxygen. For the complex to reset to the lowest state, S₀, it uses 2 water molecules to pull out 4 electrons.

The OEC active site contains a cluster of manganese and calcium, with the formula Mn₄Ca₁O_xCl_1–2(HCO₃)_y. This cluster is coordinated by D₁ and CP₄₃ subunits and stabilized by peripheral membrane proteins. Other characteristics of it have been reviewed.^[4]

Currently, the mechanism of the complex is not completely understood, along with the role of Ca+2, Cl−1, and the membrane proteins surrounding the metal cluster.^[5][6] Much of what is known has been collected from flash photolysis experiments, electron paramagnetic resonance (EPR), and X-ray spectroscopy.^[7]