Well, although Italian COMSUBIN divers are trained to use their ARO pure oxygen rebreathers down to 25 meters, they do it with the trick of starting with their lungs full of air, and some air in the counterlung, so the ppO2 never goes above 2.0 bar.Richard Pyle is famed for, among other things, surviving a chamber ride at 6.0 Bar O2 with no ill effects.
Going above 1.4Bar (New School) or 1.6Bar (Old School) or even 2.8Bar (Italian Navy RB divers) for less than 30 minutes is unlikely to kill you.
Michael
This was the safe limit for short dives when I was trained and certified in 1975. In fact my cert says "ARO down to 10m max". This was without the trick of starting with the system full of air instead of pure oxygen...
But there was a safety curve, with a time limit at any depth, from 6m (infinite time) down to 18m (10 minutes).