An oxygen refinery is a facility employed the Moneans to split oxygen from water for the purposes of life support. They are integrated with desalination plants and are equipped with corrosion resistant alloys and variable density ballasts.
In 2375, the Delta Flyer determined that the Moneans' oxygen refineries were responsible for their water planetoid's loss of containment, by increasing the ocean density and putting more pressure on the field reactor at the planetoid's center. When the Monean bureaucracy was reticent to shut down the refineries, Tom Paris and Riga attempted to destroy them from the Flyer. (VOY: "Thirty Days")