the planet's gravity, the planet's magnetic field, and biome(s).
On Earth, O2 is actually a waste byproduct created from plants. Lifeforms adapting to utilize this ever-increasing poisonous gas, and diminishing CO2 (sorry, no proper subscript), life on Earth became symbiotic between plants and animals. Plus, O2 had a benefit of higher brain processing.
If you crash land on a planet similar to earth in mass and size, and it's green, there's a chance it'll be nitro-oxy based atmosphere.
Having said that.....
There IS a higher probability of crash landing on a planet that does NOT have oxygen, and the biters would have evolved breathing something else. So, unless/until they add the option in the game configuration to start on a green/nitro-oxy planet, or one that is something else entirely. This is a great idea of a mod. Gotta try it out.
