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Oxygen is the chemical element with the symbol O and atomic number 8. It is a non-metal and a major constituent of the universe, making up about 21% of Earth's atmosphere and about 46% of the Earth's crust by mass. In its most common form on Earth, oxygen exists as the diatomic molecule O2, though it also forms other allotropes such as ozone (O3) in the upper atmosphere.

Oxygen plays a central role in biology and chemistry. Dioxygen is essential for aerobic respiration in many organisms, where it combines with organic substrates to release energy, carbon dioxide, and water. It is also highly reactive and forms oxides with most elements and compounds. Ozone, although reactive, forms a protective layer in the stratosphere that absorbs a large portion of the sun’s ultraviolet radiation.

Occurrence and production occur naturally through photosynthesis, performed by plants, algae, and cyanobacteria, which release oxygen into the atmosphere. Industrial production typically begins with the fractional distillation of liquefied air, yielding high-purity oxygen for medical, industrial, and scientific use. Liquid oxygen is widely used as an oxidizer in welding and cutting, steelmaking, chemical synthesis, and rocket propulsion (as LOX, liquid oxygen).

Historically, oxygen was discovered independently by Joseph Priestley and Carl Wilhelm Scheele in the 1770s. It was later named and described by Antoine Lavoisier, who helped establish its essential role in combustion and respiration. Stable isotopes include 16O, 17O, and 18O, with 16O being the most abundant.

Safety and handling reflect its strong oxidizing properties and its support of combustion; high concentrations can pose health and safety risks, and oxygen is stored as a compressed gas or cryogenic liquid.