A lot of the research in to how we react to reduced O2 was done about 60 years ago and may have been wrong. Some of the experiments were in a small decompression chamber with an observer breathing pure O2 sitting next to the subject. The exhaust from the observers breathing is thought to have artificially raised the concentration of O2 in the chamber and skewed the results.
There is a good section on this issue in Jean-Marie Clement's book "Dancing with the wind"
Current thinking is to use O2 from take off if the flight is going above 6,000' which with modern "mountain high" EDS equipment uses very little O2 but just enough to keep the pilot thinking well.
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