An oscillating variable displacement ring pump provides both positive and variable displacement. A housing circumscribes a pump chamber. The pump chamber encases an oscillating ring driven by a crankshaft. The crankshaft's offset shaft is located inside the pump chamber. The ring encircles the offset shaft. A bearing rotatably attached to the offset shaft rolls inside the ring. When the pump chamber is sealed, rotation of the offset shaft causes ring oscillation in the chamber. Ring oscillation creates vacuum pressure, which draws substances into pump chamber via an inlet port while pumping out substances of the pump chamber via an outlet port. A valve within the pump chamber contacts the ring and follows ring oscillation to help separate incoming substances from outgoing substances. The pump can include an adjustable internal by-pass means to control the volume and pressure of substances delivered by the pump.