The Planet With Almost Enough Oxygen is the location where Cosmo brings Timmy and Wanda as part of a tour of planets.
This planet has "almost enough oxygen" but not enough for humans nor animals. Apparently there are no plants either and.therefore no added oxygen from plants.
Although the color appears purple, that is probably caused by being in deep space. Instead the planet seems like barren grey sand, with no plants.
In the same episode when they go to a planet with strange plants, they later go to this planet indicating they are touring planets. This planet is Cosmo's choice.