Oxygen was discovered on August 1, 1774 by English naturalist and theologian Joseph Priestley. However, Priestley didn't come up with the name for his find. It was coined by the great French chemist Antoine Lavoisier.
The name for oxygen is actually a misnomer, combining the Greek words for "acid" and "producer." At the time of its discovery, oxygen was thought to be a necessary component of all acids, but that supposition has since proven to be untrue.
Oxygen's atomic number of 8 refers to the number of protons in its nucleus. The elements in the periodic table are organized according to their atomic number.