Oxygen

Oxygen was one of the primary elements required for the functioning of many carbon-based life forms.

It was vital on space stations and spacecraft to allow said life forms to breathe. (TV: The Moonbase)

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 light from space 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)