HOPON is a randomised clinical trial to discover the value of hyperbaric oxygen (HBO) in preventing jaw necrosis in long term survivors of head and neck cancer. HBO is an additional intensive treatment for patients and needs additional funding on an individual patient basis. Countered against that is the potentially very serious side effect of osteoradionecrosis. Patients having 'surgical procedures' (extractions / implants) in the jaw following radiotherapy are invited to be randomised to HBO (20 dives pre-surgery and 10 dives post-surgery) versus standard care.