The gas that keeps us alive is named for something it does not do. Oxygen means "acid-maker," and that idea turned out to be wrong.
The name came from Antoine-Laurent Lavoisier, the French chemist, in 1777. He built it from two Greek pieces. Oxys means "sharp," and by extension "acid," from the sharp taste of acids. The ending -genēs means "producer" or "begetter." Etymonline describes the coinage as a Greek dressing-up of the French phrase principe acidifiant, "acidifying principle."
Lavoisier believed every acid contained this element, so "acid-former" seemed like its defining trait. Later chemists showed otherwise. Humphry Davy, around 1812, was among those who established the point. Hydrochloric acid is the textbook counterexample: it is a strong acid with no oxygen in it at all. By then the name had stuck.
Lavoisier did not discover the gas either. Carl Wilhelm Scheele, a Swedish pharmacist, produced it around 1771 to 1772 but did not publish until 1777. He called it "fire air," since it was the only known thing that fed a flame. Joseph Priestley, an English clergyman, isolated it independently on August 1, 1774. He heated mercuric oxide with focused sunlight and named the result "dephlogisticated air."
That name reflected phlogiston theory, the old idea that burning released a substance called phlogiston. Lavoisier's work helped bring that theory down. His new vocabulary replaced the old one, and so his name survived while Priestley's and Scheele's did not.
The same naming habit produced a neighbor. In 1787 Lavoisier and three colleagues coined hydrogène, "water-producer," because the gas forms water when it burns with oxygen. Nitrogen followed in 1790, named by Jean-Antoine Chaptal for its link to nitre. Lavoisier himself preferred to call nitrogen azote, "not life."
In English, oxygen is recorded from 1790. The Greek oxys also turns up in less scientific places. It is the first half of oxymoron, a "sharp" word joined to a "foolish" one.