PLEASE CORRECT ME IF I'M WRONG, but...
Just about any "99/100% oxygen" regulator would probably deliver air. The problem is that most SCUBA "grade E" air fills are not oxygen compatible because grade-E air contains traces of hydrocarbons (compressor oil) that while not poisonous to breathe, will "contaminate" the regulator. The next time pure oxygen is used there would be a serious fire risk. And re-oxygen cleaning the reg will rapidly become so expensive that you may as well buy another set to dedicate to your other gas or gasses.
As far as I understand, this is the reason that a pure-O2 regulator is usually kept only for that task. However, I would think that using oxygen-compatible air (if you can find it and trust the source) would be fine.
Another issue... many of the O2 regs are moderate-performance setups, because you'll be breathing gently (hanging or drifting on deco) and at a depth of less than 20 feet. Perhaps they're also detuned to prevent freeflow while you're breathing your other gasses. So they may not be wonderful performers at greater depth.
Thoughts, anyone?