Habitat air composition

Speaking of
'filler' gasses, what would space habitats do about
this? I've heard that they may just use oxygen and
pretty much have no flammable materials, but I find
that unlikely. Is it? O'Neill recommended an atmosphere around 1/2 sea level pressure, with the O2 percentage doubled, and the N2 reduced accordingly. That would significantly reduce the amount of N2 otherwise needed, but the air would still be over 1/2 N2. Savage and Zubrin make far less conservative assumptions, going for even less pressure and even greater O2 concentrations.