Oxygen

Oxygen was an element with the symbol O (PROSE: Christmas on a Rational Planet) and atomic number 8. (TV: Shada)

Hydrogen and oxygen made up H₂O. (PROSE: Christmas on a Rational Planet)

Oxygen was vital on space stations and spacecraft to allow said life forms to breathe. (TV: The Moonbase) It was produced by plant life. (TV: Flesh and Stone, In the Forest of the Night) Adric, Varsh, and a number of other Alzarians used oxygen cylinders against the Marshmen, causing a change in the atmosphere too quickly for them to adapt to. They then proceeded to flood the ship with the gas, forcing the Marshmen to flee. Their Starliner had an electrolytic power supply, enabling it to create its own oxygen. (TV: Full Circle)

Vionesium reacted with oxygen to produce light and carbon dioxide in large quantities. (TV: Terror of the Vervoids)

By the 51st century, human space vessels could house oxygen factories in the form of synthetic forests, nicknamed "treeborgs". These used starlight to produce oxygen. (TV: Flesh and Stone)

Earth's atmosphere was 80% nitrogen and 20% oxygen. In the 23rd century, the Eighth Doctor had altered Mars' atmosphere with the atmospheric re-ioniser on Deimos Moonbase to make it the same consistency, saving the human colonists below. (AUDIO: The Resurrection of Mars)

There were oxygen factories on Venus during Empress Vulpina's reign. (AUDIO: Voyage to Venus)

On one day in the 2010s, trees mysteriously grew across all the land on Earth. As the Twelfth Doctor discovered, it was a defence mechanism to protect the planet from the threat of a solar flare with excess oxygen. Once the solar flare struck and the worldwide forest's work of preservation succeeded, it promptly regressed to its former state. (TV: In the Forest of the Night)

The atmosphere of Izdaal's tomb on Mars, while rich enough in nitrogen for the Ice Warriors to thrive in, also had enough oxygen for humans and Time Lords to breathe. (AUDIO: Red Dawn)

In the caves of Androzani Minor there were low levels of the element. The Fifth Doctor claimed he could go down there because he could store oxygen for several minutes. However, Sharaz Jek gave him an oxygen cylinder the same. Moreover, the Doctor's TARDIS was provided with oxygenators. (TV: The Caves of Androzani)

Qetesh didn't need oxygen to breathe. (TV: Goodbye, Sarah Jane Smith)