Are the space stations and habitation modules of Bigelow Aerospace (Genesis I & II and the planned BEAM and B330) blown up by air like balloons? Or are they unfolded by mechanical or other means? They themselves use the word "expandable" but most others say "inflatable".
Genesis I and II were technology demonstrators and inflated by increasing pressure internally with Nitrogen gas.
BEAM because it will inflate while attached the ISS will inflate with a breathable mix. Almost certainly not pure oxygen (fire risk), likely with a mix that matches the ISS atmosphere. (In fact the ISS needs large tanks of Nitrogen, since it leaks away with each spacewalk. Splitting water into Hydrogen and Oxygen is easy, getting nitrogen gas is harder.)
BA330 will have the same issues, since it will be a standalone space station, but since people will need to occupy it, most certainly a breathable mix and since you need to fill it with breathable air anyway, why not use it for two things at the same time.