Actually, the Hunt expedition (of which Hillary and Tenzing were part) did use oxygen, and they both used open circuit masks on their successful ascent. The first ascent without oxygen was made by Messner and Habeler in 1978.
The first American Expedition in an attempt to summit without O2 was in 1962 and is known as the Sayer Expedition. This was a very controversial matter. To this day that attempt is not recognized by climbing organizations as legitimate. Roger Hart, a college professor & friend of mine was on that expedition as well as the grandson of former President Woodrow Wilson, Woodrow Wilson Sayer. There is so much controversy here that the only way to the truth is to read the books by Sayer, “Four Against Everest”, and that by Hart, “The Phaselock Code” also known as “The Man Who Fell Off Everest”.