Christine,
I was put on HBO relatively late. I had a yeast infection at a seam/pocket from the part. glossectomy. Ie. something that was not healing well and I also had bleeding gums.
I did not have problems with trismus. After HBO there is no more bleeding and everything is nicely healed up. I just had a checkup with the RO on Friday, our next "date" is in half a year.

HBO is supposed to stimulate healing and the formation of new blood vessels. The will supply the tissue (and bone) with needed oxygene. (The x rays are very good at killing your capillaries and the danger is that tissue becomes necrotic).
I do not know how HBO will help with trismus, healing and rebuilding blood vessels may help... what does your surgeon say?

Best

Markus