Oxygen is #1 resource defining the survival of a human on Mars. Death awaiting in only a few minutes of its absense, Oxygen is the top priority in the list of supporting infrastructure to be built.
Colonists obtain Oxygen either from Oxygen Bottles in their Suit, or from the atmosphere in oxygenized interiors such as habitat buildings or pressurized vehicles. The latter, in turn, receive this live-giving gas from the producing part of Fluid Infrastructure (actually, Player also can tap into the distribution network directly to refill their supply direcly from a fluid fitting).