Hello and Welcome to Ask A Doctor service. I have reviewed your query and here is my advice. Radiation therapy often causes lack of saliva formation that can be due to damage to the secretory structures of the salivary gland leading to dry mouth that at times can be severe.. You can use mouth lubricant like Oralgel and use artificial saliva drops. Also you can try out Xylitol patches as it helps in maintaining a lubricated coating inside mouth. You can also use specialized mouthwash like Biotene. For hyperbaric oxygen therapy a thorough examination should be done. Discuss with your doctor about taking medicine Pilocarpine as ir helps in stimulating saliva flow. Hope I have answered your query. Let me know if I can assist you further.