- "Anywhere there’s oxygen you’re sure to find trees."
- —Daniel Jackson
Oxygen is a gas that is abundant on just about every inhabitable planet. Humans require oxygen, as well as a mixture of nitrogen to survive. (SG1: "Revisions")
It would seem that sulfur-based lifeforms such as the Gadmeer do not require oxygen as they were terraforming a planet to make it suitable for them to live on. (SG1: "Scorched Earth")