A patient over age 70 may have a normal PaO 2 around 70-80 mm Hg, at sea level. However, unless the patient is connected to a ventilator, or has a tight-fitting face mask, the appliance used to deliver oxygen to the face will always result in an inhaled FIO 2 . The body does not store oxygen. Athletes who inhale a few minutes of oxygen, and then return to the playing field, are not benefitted in any physiologic way. To give more oxygen requires a hyperbaric chamber, an expensive piece of equipment found in relatively few hospitals.
Supplemental oxygen is a drug.
Like any other drug, it has indications, contra-indications, and side effects. Supplemental oxygen is the most commonly prescribed drug in hospitals. A reduced PaO 2 is a non-specific finding. A low PaO 2 should not be used to make any particular diagnosis, including pulmonary embolism. High FIO 2 doesn't affect COPD hypoxic drive.