I spent about an hour at 14000 ft in a glider over Switzerland once. The instructor in the seat behind me behind me kept asking if I was OK. I was more than OK, I was "£$%^^ great.
We floated up and down along the sunny side of a line of cloud formed by wave lift off the mountain - fantastic. On the way down we saw climbers halfway up the mountain. They were wearing cold weather climbing gear, heavy boots etc We were wearing tee shirt and shorts. We were a bit cold but it wasn't that bad under the glass. No O2 used.