Breathing pressurized air in vacuum is not such a good idea.
The peak of Mount Everest is about 1/3 atmosphere, and base camp is in about 1/2 atmosphere. This means the partial pressure of oxygen at those locations is about 0.07 and 0.1 atmospheres, respectively. People who have not acclimated to high altitudes (by spending 2 months at base camp) will pass out on the peak (and it still sucks pretty hard even if you're acclimated).
So, it seems like pure oxygen at 0.1 atmosphere should work, although I suspect it'd be hella uncomfortable even without the vacuum, since it'd probably feel like you were trying to blow up the worlds largest balloon while standing on Mount Everest.
But it does let us move on. So you're standing in a vacuum (on the Moon, maybe) sucking on 0.1 atmosphere of pure oxygen. How long can you survive for?
You might need to keep a small amount of lung pressure (say, less than that used to blow up party balloons) to slow the egress of oxygen through the lungs.
The next problem is likely that your lungs catch fire as the O2 concentration builds up in them. One thing at a time though.