Oxygen liberated during photosynthesis comes from 

Oxygen liberated during photosynthesis comes from water.

Photosynthesis is the process of utilising carbon dioxide and water in the presence of sunlight and chlorophyll to produce glucose with the release of oxygen. 

Here is the balanced equation of the photosynthesis - 

$6{\mathrm{CO}}_2 + 12{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{sunlight}/\mathrm{chlorophyll}}{\to } {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6 + 6{\mathrm{O}}_2 + 6{\mathrm{H}}_2\mathrm{O}$ 

During photosynthesis, water molecules split to generate oxygen and hydrogen ions. These ions reduce carbon dioxide to produce carbohydrates. Remaining oxygen is expelled in the form of oxygen gas.