An Oxygen Breathing Apparatus or OBA is a 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...

. It is used primarily in firefighting. Its 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...

 is generated by chemicals

Chemical oxygen generator

A chemical oxygen generator is a device releasing oxygen created by a chemical reaction. The oxygen source is usually an inorganic superoxide, chlorate, or perchlorate. A promising group of oxygen sources are ozonides. The generators are usually ignited mechanically, by a firing pin, and the...

 contained in the green canister, which is inserted at the base of the breathing device.
The canister uses potassium superoxide

Potassium superoxide

Potassium superoxide is the chemical compound with the formula KO2. This rare salt of the superoxide ion is produced by burning molten potassium in pure oxygen...

 as both a CO₂ scrubber and an O₂ generator. A sodium chlorate

Sodium chlorate

Sodium chlorate is a chemical compound with the chemical formula . When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride...

 candle in the base generates additional oxygen when first beginning use.
As of early 2001, the US Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 has been replacing the OBA for firefighting with the SCBA. The SCBA is more cost-effective than the OBA, and also does not produce hazardous waste, a problem with the chemical oxygen generation system.