It seems like it would be impossible to have a true double blind study with something like hyperbaric oxygen. Once the chamber reaches pressure, it is very obvious to the patient.
Controls that spring to mind would be to have the chamber pressurised to a lower pressure (if you're not already familiar with hyperbaric chambers, you won't know it's at a lower pressure), or to replace the pure oxygen with normal air. Showing a dose-response relationship to pressure or oxygen concentration would go a long way towards making up for the lack of a true negative control.
Not sure if there's a lingering effect afterwards, which IIUC is what we're talking about here.