This is from the OCF main pages � you can do a search on just about any OC topic and get a comprehensive explanation.

�Osteoradionecrosis, or ORN, is the most serious possible complication facing the oral cancer patient. A condition of the non vital bone in a site of radiotherapy (RT), Osteoradionecrosis is bone that has died as a complication of radiotherapy. Because radiation works to destroy cancerous cells through the deprivation of oxygen and vital nutrients, it inevitably destroys normal cells as well, damaging small arteries and reducing circulation to the area of the mandible. Not an infection itself, it is the bone�s reduced ability to heal and the resulting lesions, pain and fragility. Insufficient blood supply to the irradiated areas decreases the ability to heal, and any subsequent infections to the jaw can pose a huge risk to the patient. Though it is possible to develop spontaneously, ORN most frequently occurs when an insult to the bone is sustained in the irradiated area, such as related subsequent surgery or biopsy, tooth extractions or denture irritations.�

In the same article they talk about HBO -- Hyperbaric Oxygen Therapy.

�There is, at least, general agreement that HBO treatments raise tissue oxygen levels which can affect the healing process and give greater infection control. Many studies show that HBO treatments are more effective than penicillin with patients showing a significantly lower rate of necrosis. One of the advantages to this treatment is that it is non-invasive and painless. Patients are placed in pressurized chambers while pure oxygen is compressed into the chamber. The treatments raise tissue oxygen levels and causes proliferation of blood vessels.�

HBO therapy is normally recommended before any dental extractions. Usually 20 dives before the dental work and 10 afterwards.