An oxygen tank is a storage vessel for oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, which is either held under pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 in gas cylinder

Gas cylinder

A gas cylinder is a pressure vessel used to store gases at above atmospheric pressure. High pressure gas cylinders are also called bottles. Although they are sometimes colloquially called "tanks", this is technically incorrect, as a tank is a vessel used to store liquids at ambient pressure and...

s or as liquid oxygen

Liquid oxygen

Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...

 in a cryogenic storage tank.
Oxygen tanks are used to store gas for:

• industrial processes including the manufacture of steel
  Steel
  Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
  
   and monel
  Monel
  Monel is a trademark of Special Metals Corporation for a series of nickel alloys, primarily composed of nickel and copper, with some iron and other trace elements. Monel was created by David H. Browne, chief metallurgist for International Nickel Co...
  
  
• oxyacetylene welding
  Welding
  Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
  
   equipment and some gas cutting torches
• use as the liquid rocket propellants
  Liquid rocket propellants
  The highest specific impulse chemical rockets use liquid propellants. This type of propellant has a long history going back to the first rockets and is still in use, for example in the Space Shuttle and Ariane 5.-Early development:...
  
   for rocket engine
  Rocket engine
  A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
  
  s
• medical breathing gas
  Breathing gas
  Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...
  
   at medical facilities and at home
• breathing at altitude in aviation, either in an uncontrolled decompression emergency, or constantly (in the case of unpressurized aircraft)
• oxygen first aid kits
• gas blending
  Gas blending
  Gas blending or gas mixing is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox.Filling cylinders with a mixture of gases has dangers for both the filler and the diver. During filling there is a risk of fire due to use of oxygen and a risk of explosion...
  
   for creating diving breathing mixes such as nitrox, trimix and heliox
  Heliox
  Heliox is a breathing gas composed of a mixture of helium and oxygen .Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of...
  
  
• open-circuit scuba set
  Scuba set
  A scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving. It is much used for sport diving and some sorts of work diving....
  
  s - mainly used for accelerated decompression in technical diving
  Technical diving
  Technical diving is a form of scuba diving that exceeds the scope of recreational diving...
  
  
• some types of diving rebreather
  Rebreather
  A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...
  
  : oxygen rebreathers and fully closed circuit rebreathers

Breathing oxygen is delivered from the storage tank to the users by use of the following methods: oxygen mask

Oxygen mask

An oxygen mask provides a method to transfer breathing oxygen gas from a storage tank to the lungs. Oxygen masks may cover the nose and mouth or the entire face...

, nasal cannula

Nasal cannula

The nasal cannula is a device used to deliver supplemental oxygen or airflow to a patient or person in need of respiratory help. This device consists of a plastic tube which fits behind the ears, and a set of two prongs which are placed in the nostrils. Oxygen flows from these prongs...

, full face diving mask

Full face diving mask

A full-face diving mask is a type of diving mask that seals the whole of the diver's face from the water and contains a mouthpiece or demand valve that provides the diver with breathing gas...

, oxygen tent

Oxygen tent

thumb|300pxAn oxygen tent consists of a canopy placed over the head and shoulders or over the entire body of a patient to provide oxygen at a higher level than normal.Oxygen tents are also used by athletes-see altitude tents.-External links:*...

, and hyperbaric oxygen chamber.

Contrary to popular belief scuba divers

Scuba diving

Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

 very rarely carry oxygen tanks. The vast majority of divers breathe air or nitrox stored in a diving cylinder

Diving cylinder

A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....

. A small minority breathe trimix, heliox

Heliox

Heliox is a breathing gas composed of a mixture of helium and oxygen .Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of...

 or other exotic gases

Breathing gas

Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...

. Some of these may carry pure oxygen for accelerated decompression or as a component of a rebreather. Some shallow divers, particularly naval divers, use oxygen rebreathers or have done so historically.

As liquid oxygen at atmospheric pressure

Atmospheric pressure

Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

 boils at -183°C / -297°F a liquid oxygen tank must store the oxygen below that temperature and be a good thermal insulator.

Oxygen is rarely held at pressures higher than 200 bar / 3000 psi due to the risks of fire triggered by high temperatures caused by adiabatic heating

Adiabatic process

In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...

 when the gas changes pressure when moving from one vessel to another.

All equipment coming into contact with high pressure oxygen must be "oxygen clean" and "oxygen compatible" to reduce the risk of fire

Fire

Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....

. "Oxygen clean" means the removal of any substance that could act as a source of ignition

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

. "Oxygen compatible" means that internal components must not burn readily or degrade easily in a high pressure oxygen environment.

In some countries there are legal and insurance

Insurance

In law and economics, insurance is a form of risk management primarily used to hedge against the risk of a contingent, uncertain loss. Insurance is defined as the equitable transfer of the risk of a loss, from one entity to another, in exchange for payment. An insurer is a company selling the...

requirements and restrictions on the use, storage and transport of pure oxygen. Oxygen tanks are normally stored in well ventilated locations, far from potential sources of fire and concentrations of people.