In photosynthesis, oxygen is liberated during

In photosynthesis, oxygen is liberated during Hydrolysis of water.
Two water molecules are oxidized by the energy of four successive charge-separation reactions of photosystem II to yield a molecule of diatomic oxygen and four hydrogen ions.
The electrons yielded are transferred to a redox-active tyrosine residue that is oxidized by the energy of P680⁺. In light, the electron acceptor is reduced and oxygen is evolved.