O2 stands for oxygen.
CO2 stands for carbon dioxide.

Respiration uses oxygen produced during photosynthesis.
Photosynthesis uses the carbon dioxide produced in respiration.
This is the oxygen-carbon dioxide cycle.

Plants need carbon dioxide to perform the photosynthesis.
During photosynthesis plants produce oxygen.
Animals need oxygen to breathe.

Plants and animals are connected by the oxygen-carbon dioxide cycle.

Carbon, oxygen and you
Via It’s Okay To Be Smart