Emergency O2 is exactly what you need selected at these altitudes. When you have the mask on it won't be a continuous flow anymore, but it just assists you breathing with a small positive pressure.
Back to the original question - also consider decompression sickness before you plan to continue with a high (>F250) cabin altitude. Ideally you should breathe 100% oxygen before going to those altitudes to get the nitrogen out of your blood. The risk increases significantly above F250, and also increases with the time of exposure, age and obesity of the person, and the amount of activity at altitude.
Be careful up there!
Joe