氧（IUPAC名：oxygen ），化学符号：O（拉丁语：Oxygenium ），化学元素，氧的原子序数为8，相对原子质量为15.9994。在元素周期表中，氧是氧族元素的一员，它也是一个高反应性的第2周期非金属元素，很容易与几乎所有其它元素形成化合物（主要为氧化物）。在标准状况下，两个氧原子结合形成氧气，是一种无色无臭无味的双原子气体，化学式为O₂。如果按质量计算，氧在宇宙中的含量仅次于氢和氦，在地壳中，氧则是含量最丰富的元素。氧不仅占了水质量的89%，也占了空气体积的20.9%。

中文旧译作氱 （拉丁语：Oxygenium），法语：Oxygène，英语：Oxygen)，字源学来说是根源于希腊文的成酸物质（酸oxus+成genes）或「产生酸的东西」。

Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetal and oxidizing agent that readily forms compounds (notably oxides) with most elements. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O
2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. However, oxygen levels in the atmosphere are trending downward globally, possibly because of fossil-fuel burning. Oxygen is the most abundant element by mass in the Earth's crust as part of oxide compounds such as silicon dioxide, making up almost half of the crust's mass.