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. 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. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S to S, with S the most reduced and S the most oxidized. Photons trapped by photosystem II move the system from state S to S. S is unstable and reacts with water producing free oxygen. For the complex to reset to the lowest state, S, it uses 2 water molecules to pull out 4 electrons.
The OEC active site contains a cluster of manganese and calcium, with the formula MnCaOCl(HCO). This cluster is coordinated by D and CP subunits and stabilized by peripheral membrane proteins. Other characteristics of it have been reviewed.
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. Much of what is known has been collected from flash photolysis experiments, electron paramagnetic resonance (EPR), and X-ray spectroscopy.