I have seen it on rare instances where, say the captain needs to use the washroom, per some company procedures the F/O would have to wear the oxygen mask while the captain is gone, for safety I would believe.
I know this as we have been called out to aircraft where the O2 mask has been used, and the crewmember can't stow it.
Obviously this would not apply in this situation, but nice to know info.
[Edited 2004-01-16 23:21:57]
Break ground, and head into the wind. Don't break wind and head into the ground.