Well, to answer that question, you would first want to know who first discovered oxygen, and there is no simple answer to that question! There are three people to whom discovery of this can be ascribed: Carl Wilhelm Scheel, Joseph Priestley, and Antoine Lavoisier. Scheele produced O2 (he called it “fire aire”) from mercuric oxide (HgO) in 1772, but the result wasn’t published until 1777. Meanwhile, in 1774 Priestley produced O2 (he called it “dephlogisticated air”) using a similar experiment, which was published in 1775. Lavoisier claimed to have independently discovered the gas, and was in fact the first to explain how combustion worked via quantitative experiments, leading to the principle of Conservation of Mass, and ultimately disproving the entire idea of phlogiston. Whew. So Scheel found it first, but didn’t report it; Priestley reported it first, but didn’t have the explanation correct; and Lavoisier was last, but nailed it. Who would you give credit to?