Here's why:

* Oxygen is produced in the light reactions. The light-dependent reactions of photosynthesis use light energy to split water molecules. This process releases oxygen as a byproduct.

* Dark reactions (Calvin cycle) use carbon dioxide. The dark reactions use the energy produced in the light reactions to convert carbon dioxide into sugar (glucose). This process does not involve water splitting or oxygen production.

In summary: The light reactions are responsible for oxygen production, while the dark reactions are responsible for carbon dioxide fixation and sugar synthesis.