This forum's weird rules prevent me from posting links since I'm now. So anyone interested in actual clinical data I am trying to link below will need to go digging for themselves.

In 2015 Efrati published a study on HBOT for blunt mTBI people without PTSD. The results echo Teaaquinn's improvements.

Furthermore, the study addresses the flaws of prior studies showing no benefit over placebo (as Mark_in_Idaho mentioned). Basically, what was previously thought of as placebo was actually therapeutic dosing of oxygen.

Google this: PMC4499900

Note that isn't reasonable to attempt a placebo for the patient, so a double blind study isn't easy to create. Note that in the study above, objective SPECT scans (and neuropsyc testing to a degree) provide strong evidence without a double blind setup.

Since this post was last active, Dr. Harch also published results similar to Efrati's linked above:
Google this: PMC5674654

Regarding getting worse after HBOT, that is extremely unlikely. Dr. Harch's paper here will shed some light on anyone with enough brain power to read all of it...
Google this: dosage-hyperbaric-oxygen-chronic-brain-injury

Lastly, Dr. Figueroa has these two informative papers:
Google this: 5-Clinical-Results-in-Brain-Injury-Trials-Another-Perspective

Google this: Figueroa-Wright-Neurology-HBOT-Evidence.pdf

Bottom line is that HBOT works for many people. Whether it's worth the cost to one is their own decision. I've seen it as low as $100 per 1 hour session. Rather than look at the cost of it, a better question is "can you afford not to?"