The primary role of oxygen in cellular respiration is to

The primary role of oxygen in cellular respiration is to make energy.

Explanation:

When we breathe our lungs inhale and exhale air. This is the way our body has to get in what we need the most: Oxygen. This is the star of the cellular respiration.

Cellular respiration is a process where the cells are able to make energy. Oxygen is needed in this process for organisms who have an aerobic respiration. Other organisms like some bacterias have an anaerobic respiration which does not need oxygen. In those cases, they have other mechanisms of making energy.  

So we must talk about Aerobic Respiration, which uses Oxygen. Every cell in our body is engaged in this process. Cells use this element and glucose. Glucose is a type of sugar found in foods like fruits, cereals, sweets and many more. It has a lot of different steps. It starts converting glucose into ATP (adenosine triphosphate), or cellular energy, and carbon dioxide.

the trait of personality falls in the middle of the diagram because personality is a mixture of ones nature and environment.