Along the trail of Mt. Fuji the mountain lodges sold canned oxygen for people who are having trouble adjusting to the changing altitude. We never had a need for it so we saved a few hundred yen by buying at the bottom of the mountain.
I didn't notice anything substantially different other than a slightly plastic taste in the back of my throat. Next time we climb Mt Fuji (yes, we'll probably do it again) we will use it at the top to see if we feel a difference.