Oxygen is used for breathing on ships, without which pilots will die. It is not a precious commodity due to its abundance.
A typical human being consumes over 12,000 liters of Oxygen (O2) daily. Oxygen, usually mixed with a neutral gas such as nitrogen, is absorbed through the tissues of the lungs and then diffused into the bloodstream, where it is carried throughout the body. All humans require a constant supply of Oxygen or they will perish within minutes.
Oxygen is cheapest at Planet Stuttgart (3 credits) Ulm Border Station (4 credits), and Planet Curacao (2 credits), which is cheaper than the 5 or more credits per unit Oxygen costs everywhere else, including on other planets. The best place to sell it is in the Edinburgh system, particularly Luxury Liner Shetland near Planet Gaia where Oxygen sells for 40 credits, allowing a 36 credit profit per unit or Aberdeen Border Station, where it sells for 35 credits per unit.