An Oxygen Garden is used to supply breathable atmosphere aboard space stations and other space vessels. It will also refill empty Oxygen Tanks and clean/recharge Carbon Dioxide Scrubbers, when they are placed in the empty slot (with a Water Tank) inside the Oxygen Garden. If you go to inside view, it looks like a lot of green plants with water tubes around it. It is very useful if you have a large space stations with astronauts in them.
This part is unlocked after successfully completing Mission 27.