This is kind of similar to the reverse process in plants where CO2 is taken up and O2 is released. The oxygen in the O2 molecules is not derived from the CO2 taken up for photosynthesis, but instead from the H2O molecules that are also involved in the photosynthesis process. The oxygen molecules in the CO2 molecules are mainly used for the synthesis of sugars, while the photosynthesis process involves water being split into separate hydrogen and oxygen ions. The hydrogen ions are used as part of the process for splitting the CO2 into carbon and oxygen, while the oxygen ions are released from the plant. The origin of the released O2 from the H2O instead of the CO2 has been determined through controlled experiments where the inbound H2O and CO2 contained a different isotope of oxygen.