There are 14 peaks in the world over 8,000 metres. All have been climbed without supplementary oxygen, after acclimatisation.
The death rate for climbers attempting them is horrendous:
List of deaths on eight-thousanders - Wikipedia, the free encyclopedia
There have been many feats of outstanding skill on these mountains. But climbers call the high altitude the death zone, where if you stop to help someone in trouble you will both end up dead. And there are lots of bodies still on the mountains that cannot be brought down.
Wikipedia have a good article on the effect of high altitude on humans with many references that can be followed:
Effects of high altitude on humans - Wikipedia, the free encyclopedia