Need help with inverter
Boost converter is your friend
https://en.m.wikipedia.org/wiki/Boost_converter

Also have you thought about actually making pure oxygen? You could hook up a water electrolyzer to the existing 12v wiring and just vent the hydrogen or even better turn the hydrogen back into water from low concentration atmospheric oxygen and put it back into the electrolyzer. Such setup could be even more efficient than the machine you have.