Lamp oil stills can be found in the Chemist's House in Rimmington and near Miltog's stall in Dorgesh-Kaan. Its one and only function is to produce lamp oil. To do this, use swamp tar on the lamp oil still, then use an empty oil lamp, oil lantern, or a bullseye lantern on the oil still to fill it.
Lighting a bullseye lantern inside the chemist's shop in Rimmington is one of the medium Falador tasks.