Oxygen/Legends

"Relax, and you can breathe. This symbiote filters oxygen from the sea. It can last for weeks under water."

- Cilghal to Leia Organa Solo

Oxygen (O) was the elemental gas that many beings breathed to survive, most notably Humans. Such species used lungs to take in the nourishing gas. It was, however, poisonous to Celegians, Kel Dor, Gand, and various other species. Oxygen was found on many inhabited planets and moons throughout the galaxy. It was highly reactive; considered a reducing agent. Oxygen was present as oxides in Tibanna gas on Bespin, and was vital to the creation of silica by bonding with silicon.

Most forms of life relied upon oxygen compounds for respiration. Oxygen released energy when exposed to certain enzymatic agents, and organisms utilized this energy in their metabolic processes. Oxygen was also necessary for the creation of water, which few organisms could exist without.

The Fabrennix Oxygen Extractor was a device that was often used to extract oxygen from underground water as well as water vapor sources. Using a meter-long probe, it dug into the surface of a planet in order to collect individual molecules of water so that its operator would have enough to drink. The Mon Calamari also used Oxygen Symbiotes in order to breathe underwater. These symbiotes were able to filter pure oxygen from water, transferring it into the operator's lungs through their mouth and nose.

Behind the scenes
In real life, oxygen  (IPA: /ˈɒksidʒən/) is a chemical element with the chemical symbol O and atomic number 8. On Earth it is usually bonded to other elements covalently or ionically. The abundance of significant amounts of free oxygen in Earth's atmosphere is thought to be the effect of millions of years of plant life processes.

Appearances
While oxygen presumably appears on nearly all inhabited planets, the following is only a list of sources in which oxygen is specifically mentioned: