The oxygen in the CO₂, along with the carbon in the CO₂ and the hydrogen in the H₂O, are converted by the plant into sugar, which contains carbon, hydrogen, and oxygen.

You can read an answer that we have on our database showing the way in which photosynthesis works. There on the third paragraph you will see how the oxygen in carbon dioxide is combined with water, in order to get sugar and free oxygen.

photosynthesis

Photosynthesis is actually the name for a series of interrelated processes, the light-dependent reactions and the light-independent (or dark) reactions. In a nutshell, during the light reactions, energy from photons is converted into chemicals called ATP and NADPH through a series of chemical reactions. One of the chemical reactions requires electrons from an outside source. The plant uses water to provide these electrons via something called a oxidation- reduction (redox) reaction, and a byproduct of this reaction is oxygen. The dark reactions use the oxygen and carbon in carbon dioxide and the NADPH and ATP from the light-dependent reactions to create sugars and starches for the plant which will be used in other reactions in the plant.