The source of oxygen liberated as byproduct during photosynthesis is

During the light-dependent reactions of photosynthesis, solar energy drives the photolysis (splitting) of water (${\text{H}}_2\text{O}$) molecules within the thylakoid membranes of chloroplasts:
$$2{\text{H}}_2\text{O}\stackrel{\text{light}}{\to }4{\text{H}}^{+}+4e^{-}+{\text{O}}_2\uparrow$$
This process releases oxygen gas (${\text{O}}_2$) into the atmosphere as a byproduct. Heavy-isotope tracer experiments have confirmed that this liberated oxygen comes entirely from water molecules, not from carbon dioxide (${\text{CO}}_2$).