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Liquid oxygen, commonly abbreviated LOX, is the liquid form of elemental oxygen. It is produced by cooling oxygen gas to its boiling point at standard pressure, which is -183°C (-297°F). LOX is pale blue, paramagnetic, and remains a cryogenic liquid under ordinary storage conditions. It has a density of about 1.14 g/cm³ at its boiling point and is a highly reactive oxidizer that can intensify fires.

LOX is produced industrially by fractional distillation of liquid air. In this process, air is compressed and cooled so that its components condense at different temperatures, allowing oxygen to be separated as a cold liquid. It is stored in specialized cryogenic insulated dewars and transported in vacuum-insulated containers. Handling LOX requires oxygen-compatible equipment and strict safety measures; leaks can enrich the surrounding atmosphere with oxygen, markedly increasing fire hazards. Safety systems typically include pressure-relief devices and continuous oxygen monitoring.

Common uses of liquid oxygen include rocket propulsion, where LOX serves as the oxidizer in combination with fuels such as kerosene or hydrogen. It is also used in steelmaking, metal processing, and various chemical manufacturing applications. In medical and industrial settings, LOX can be evaporated to produce gaseous oxygen for patient support or process needs, with storage in cryogenic tanks.

Historically, the liquefaction of oxygen occurred in the late 19th to early 20th century, enabling practical applications in industry and rocketry. Today LOX remains one of the most widely used cryogenic oxidizers, valued for its high oxygen content and large energy density.