The oxygen evolved during photosynthesis comes from water...

During non-cyclic photophosphorylation, the PS-II helps in the photolysis of water molecules which evolves oxygen as a by-product. For the photolysis of water, some cations (like $M{g}^{+ 2}andM{n}^{+ 2}$) and anions (like $C{l}^{-}$) are involved. The proton generated in this process is used to make the proton gradient for ATP synthesis.

Manganese, chlorine, and calcium help in the photolysis of water during the light reaction of photosynthesis.