We went to a major teaching hospital with 2 large hyperbaric chambers that treat up to 10 people in each. My son was seen by a hyperbaric medicine MD before and after each treatment as was everyone else. There are some "fake" units that carry high risk and don't yield the excess oxygen levels.
The theory for this with Crohn's (perianal usually) is that the hyperperfusion of oxygen aides in healing. Studies also show some anti-TNF properties from HBO2. We primarily used it for wound healing since the abscess was unearthed, had been there a long time and was huge. There were people in their for a large variety of reasons. Most is for diabetic wounds, some for carbon monoxide poisoning, some brain cancer (it really helps people to keep symptoms at bay from inflammation around the tumor). We talked with so many people helped by this therapy. It is much more expensive in this setting but the care was great. In fact, my son's doctor in the chamber was a pediatrician for 25 years and then became a wound care doctor and then got his hyperbaric medicine board certification.
If you go this route, make sure you are monitored and you are doing it in a large chamber. Then you will know you are in a good place. Also, a board certified physician in hyperbaric medicine overseeing things is a must!