Hyperbaric oxygen therapy (HBOT) involves giving oxygen at high concentrations to help various diseases, including cancer. It is currently being used for osteoradionecrosis.
One article (1) indicated that the US Food and Drug Administration has issued a statement that HBOT has not been proven to be effective in the treatment of cancer, nor can it treat autism and diabetes, which is what some practitioners are doing to treat these diseases.
On the other hand, the American Cancer Society indicates that HBOT therapy can be used as mainstream therapy for the treatment and prevention of osteoradionecrosis, which is bone damage caused by radiotherapy. There is also some research indicating that it can be a therapy for soft tissue injuries caused by radiation therapy.
Source: 1.FDA Issues Deceptive Statement about HBOT (Hyperbaric Oxygen Therapy). https://healthimpactnews.com/2013/fda-seeks-to-restrict-hyperbaric-oxygen-therapy-used-in-cancer-treatment/. Accessed 9/9/16.