There ain't no such thing as a free lunch was often a plot point in the works of Robert Heinlein, particularly in regards to air. We know that Air in International Space Station comes at price, in Heinlein's work even the largest stations required regularly payments for oxygen, with penalties usually around loss of access to air.
On Earth oxygen is made available by the biosphere (plants). Nowhere else in our solar system is free (unbound) oxygen readily available. There are some processes that generate Oxygen such as Chemical oxygen generators.
Is there any place in the solar system where oxygen would be a waste product of industry, effectively making oxygen free (without additional cost) to the inhabitants?