An Oxygen Garden is used to supply breathable atmosphere aboard space stations and other space vessels via producing oxygen with onboard plants and using water supplies to do so. It will also refill empty Oxygen Tanks and clean/recharge Carbon Dioxide Scrubbers, when they are placed in the empty slot 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.