In theory, yes, but it never happens for the following reason. The oxygen that the plant produces actually comes from the water, not the carbon dioxide, that the plant is using. We know this because if you replace the oxygen in the carbon dioxide with a radioactive isotope, the radioactive oxygen will get incorporated into the plant, not released into the air. However, if you make the oxygen in the water radioactive, then the plant will give off radioactive oxygen.

However, loose hydrogen inside of a plant will make the plant's internal fluids very acid, since loose protons in water are what make something acid. This would kill the plant. Therefore a plant will never strip the hydrogens off of water without carbon to bind them to, which is why, in practice, plants only release oxygen when they have carbon dioxide to use.