Be careful before you start blowing off the O2 mask above FL250 -- the regulation (14 CFR) has changed (will change, effective Jan 9), but your company's procedures may not have caught up yet.
Even though I won't be required to wear the mask above FL250 by 14 CFR, I will still be required by my company's FOM - - until we can get it changed.
The truth only hurts if it should.