In Planetoid 3, if you choose any organic based race, your people obviously will require oxygen to survive. Your first planet automatically starts with trees and full oxygen, but you can easily lose oxygen and die if your planet becomes overpopulated or all your trees are killed. Other uninhabited worlds may or may not have trees and oxygen. If they don't, (see Oxygenless Planet) then the player will have to send a Planter ship to spread trees around the world, and thus produce oxygen for your people

Oxygen can be viewed on each planet's stats bubble, see image below:

Planet stats