oxygen use in gliders is routine
At Aboyne, the Deeside Gliding Club, oxygen is carried, used with a mask or with canula. Above 12,000 in mountain wave, flying solo, I decided to turn it on; with a choice of settings of either 2 or 4, thought it would be a good idea to put flow half way, assuming that would give 3 per minute, or however the flow is calculated. Actually, this gave me no oxygen at all! so from there to 15,000 feet I was breathing the ambient air.
The safety system at the gliding club is that once you tell them you are going on oxygen, they radio you every five or ten minutes to find out if you are still talking sense....I was sharply told that it would be a good idea to descend right away, and my understanding of the system was given a thorough overhaul before the next attempt at diamond height. Got it eventually, at 20,300'. But don't really care to tempt fate again, so having the gain of height award I now settle for sensible heights, no more than 12,000, thank you.
But gliders fly up to 50,000' in mountain wave these days. Pressurised systems and suits, I believe.