Dear Octo:
Oxygen is not considered in dive calculations, since it is metabolized and is always in low concentration when compared to the inert gases (nitrogen, helium, etc). Their lack of inclusion in the models of Buhlmann and Schreiner is well noted.
The endobiotic gases (water, carbon dioxide and oxygen) comprise a very small percentage of the tissue gases. For example, the approximate partial pressures of these at body temperature are water = 47 torr, carbon dioxide = 45 torr and oxygen = 40 torr for a total of 127 torr. The partial pressure of nitrogen in the tissues at sea level will be approximately 570 torr. Table algorithms are always adjusted to take into account these gases; their influence become progressively small as one goes to deep.
The situation at high altitude is quite different, however. In this case, the role of the inert gases becomes progressively larger. In fact, the commonly known fact that blood boils at altitude is ascribable to these partial pressures. Even in the absence of any inert gas, these endobiotic gases will be present and contribute to bubble growth. This is not a problem for recreational divers but it is a problem for aviators and astronauts.
Dr. Deco