Abstract: A hydraulic pump unit for pumping oil from oil-bearing strata which includes a frame slidably mounted on a stationary push-off tray and a hydraulic pumping unit mounted on the frame, which pumping unit further includes a pair of hydraulic cylinders supporting a head assembly. The head assembly is carried by a traveling mast slidably positioned in a vertically-oriented, stationary mast mounted on the frame, and includes a pair of rotatably-mounted chain rollers carrying a pair of chains attached to the stationary mast at one end and a bridle at the opposite end, which bridle also carries a polish rod extending into the well. Extension of the pistons in the hydraulic cylinders according to a selected hydraulic fluid flow control system causes the head assembly and traveling mast to move upwardly and the rollers to rotate with respect to the head assembly and extend the chains and polish rod upwardly to effect pumping of oil from the well.