In oxygenic photosynthesis, photosystem II (PSII) uses light energy to extract electrons ultimately from water.

Its reaction-center chlorophyll is commonly called P680 because it absorbs strongly near $680,\mathrm{nm}$. After excitation, P680 donates an electron to its acceptor side and becomes the strongly oxidizing species $\mathrm{P680^+}$.

PSII replaces that missing electron through the oxygen-evolving complex, a metal-containing catalytic center that accumulates oxidizing equivalents and removes electrons from water.

The overall water-oxidation half-reaction is

$$\boxed{2H_2O\rightarrow O_2+4H^++4e^-}.$$

Oxygen comes from water

The molecular oxygen released by oxygenic photosynthesis is produced by oxidation of water, not by splitting carbon dioxide.

The four electrons from two water molecules replace electrons removed from PSII reaction centers. The four protons are released on the thylakoid-lumen side, contributing to the transmembrane proton difference that can later be used for ATP synthesis.

The coupling can be summarized as

light excites P680
      ↓
P680 donates electron downstream
      ↓
P680+ must be reduced again
      ↓
water supplies replacement electrons
      ↓
O2 + lumenal H+ are produced

Water oxidation requires repeated photochemical events

Water oxidation is a four-electron process. The catalytic complex stores oxidizing equivalents over repeated photochemical events until enough have accumulated to form an O–O bond and release $O_2$.

This avoids requiring one concerted four-electron transfer from water in a single reaction-center excitation.

PSII therefore performs two coupled functions: it injects light-energized electrons into the photosynthetic electron-transfer chain and uses the resulting oxidizing power to make water the ultimate electron donor of oxygenic photosynthesis.