Oxygen is a clear, odourless gas and is part of all body functions, since the body requires a continual supply to survive. Many bacteria and other infection-causing organisms are conversely anaerobic, meaning they live in a low-oxygen environment.

The term oxygen therapy describes treatments that use forms of the oxygen molecule to destroy infections and rejuvenate body tissues. Although these therapies are considered useful by some PHAs, there are significant risks associated with their misuse. Oxygen therapies should be used with the aid of a knowledgeable and experienced practitioner. Users should proceed with extreme caution, however, since several people have died from the misuse of oxygen therapies.

Hyperbaric oxygen therapy (HBO) is probably the most well-documented form of oxygen therapy in the field of HIV. People receive it by sitting in a chamber that increases the air pressure and proportion of oxygen they breathe in. HBO chambers are primarily used to treat carbon monoxide poisoning and decompression sickness in divers who have been under water too long. They're also used to treat tissue damage caused by burns and radiation by providing an increased amount of oxygen to the damaged tissue. At least one study suggests that HBO therapy may be useful in treating the damage caused to lung tissue by PCP (Pneumocystis Carinii Pneumonia). Other studies suggest that HBO may be effective in treating HIV-associated fatigue and might even work to reduce viral load. HBO chambers are available in large hospitals in most urban centres. Arrangements to access one have to be made through a doctor.

Many PHAs take antioxidant supplements like N-acetyl cysteine (NAC) or vitamins C and E to counteract oxygen free radicals in the body. Although they are natural byproducts of the body's metabolism, these free radicals may be increased in people with HIV, potentially damaging the body's cells and decreasing the ability of those cells to resist infection. Oxygen therapies like those described above actually increase the number of oxygen free radicals in the body. Antioxidant supplements are usually recommended for those who use oxygen therapies.