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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)
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)
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)
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)