By being the molecules to Krebs cycle and electron transport
Oxygen is the only molecules to grab the reactive electron from NADH or FADH2 via hydrogen transporter molecules and become water..that is why glucose reaction creates CO2 and H20(water)
Without oxygen molecules enough for aerobic reaction, there is no molecules could catch the hydrogen reactive(reactive electron) from NADH and FADH2.So the NADH and FADH2 accumulated and halt Krebs cycle and halt the reaction from pyruvate to acetyl-CoA
Besides the most valuable energy resources is in electron transport which creates up to 28 ATP-32 ATP.
Oxygen is the key for generating energy in cellular respiration.
The cellular respiration is a complex biological process that breaks down sugars, fat and proteins and transforms these materials into energy for the functioning of the organism. Oxygen is needed at the end of this process, when electrons liberated in the respiration are transported through the inner membranes of the cells, and the oxygen "attracts" these electrons and makes possible the production of great quantities of chemical energy in the said membranes.