What is oxygen used for? Is it an element?

Oxygen forms a molecule (O2) of two atoms which is a colorless gas at normal temperatures and pressures. Oxygen is the most abundant element in Earth's crust, and after hydrogen and helium, it is the third-most abundant element in the universe. 

Uses

Medical use

Oxygen is an essential medicine. Healthcare professionals use oxygen to treat respiratory illnesses like COVID-19 and pneumonia. Oxygen is also essential for surgery and trauma. Vulnerable groups like the elderly, pregnant women, and newborns need oxygen on a regular basis.

Water treatment

Pure oxygen is used in ozone production. Ozone is very useful in a variety of everyday situations, from removing odors to providing clean, quality drinking water from the tap. 

Metal production

Oxygen significantly increases gold processing and ore flow, reducing the cost of cyanide and waste products. The steel industry is the largest user of oxygen. Industrial oxygen generators are also used in the production of other metals such as copper and lead. A continuous supply of large amounts of oxygen is essential for the safety of miners. Oxygen production directly on-site using oxygen generators is an efficient, and safe way to provide such a supply.

Safety

The NFPA 704 standard rates compressed oxygen gas as nonhazardous to health, nonflammable, and nonreactive, but an oxidizer. Refrigerated liquid oxygen (LOX) is given a health hazard rating of 3 (for increased risk of hyperoxia from condensed vapors, and for hazards common to cryogenic liquids such as frostbite), and all other ratings are the same as the compressed gas form.

Toxicity

Oxygen gas (O2) can be toxic at elevated partial pressures, leading to convulsions and other health problems. Oxygen toxicity usually begins to occur at partial pressures more than 50 kilopascals (kPa), equal to about 50% oxygen composition at standard pressure or 2.5 times the normal sea-level O2 partial pressure of about 21 kPa. It is not a problem except for patients on mechanical ventilators, since gas supplied through oxygen masks in medical applications is typically composed of only 30–50% O2 by volume (about 30 kPa at standard pressure).