Many people talk about climbing without oxygen, yet few actually do it. Even the sherpas will usually use oz.
In 1999, we climbed Everest with a Sherpa named Babu.
He stayed on the summit for 22 hours with no supplementary oxygen. That year, no other sherpa climbed on the south side without oz.
Babu had an extraordinary genetic ability even for a sherpa (together with a strong mind) to climb without oxygen. In addition, a safety net had to be built around Babu for his attempt.
Babu had then a history of seven Everest summits without oxygen. The attempt was prepared for a full year with added sherpas, ropes and oxygen for rescue. During the night of the attempt, we stayed at C4 with additional rescue sherpas positioned at C2. We monitored Babu all through the night, continuously asking him his name, age, number of children and other things, to check him for signs of AMS and HACE.
Even with all these precautions in count, Babu's attempt was still highly dangerous (and actually looked pretty bad for a while). Babu made it against all odds and Nepals first international climbing hero was born. A few years later, Babu sadly died in a crevasse at C2.
Do not attempt Everest without oxygen, unless you are an experienced +8000-meter/26000 ft climber, with at least one previous experience at 8500-meters/28000 ft without oz. Your sherpa should carry a spare oxygen bottle for emergency. And remember to drink plenty of fluids.