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