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 crank assembly. The ring encircles an end of the crank assembly. The crank assembly includes an annular spacer that rolls inside the ring. When the pump chamber is sealed, rotation of the crank assembly 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.