John and Eric are right about pre-breathing preventing decompression
sickness (precipitation of N2 in fluids and joints due to decreased
solubility at altitude) but it does not prevent hypoxia,

Even worse: Pure oxygen is poisonous at atmospheric pressure and should
not be breathed over an extended time. You'll find more about this in
advanced scuba diving books.

Stefan

Yes, when I was in school there was a lab in my department that showed a
hyperoxic environment (100% O2 for 24 hrs) can induce severe airway
epithelial damage and used such a model to study how the epithelium responds
to injury.

Tim