A chain saw is provided with an oil pump for supplying oil to the saw chain. The pump includes a piston immersed in a chamber formed in the housing of the saw and rotatable by a gear rigidly connected to the piston and rotated by another gear driven by a motor. The gear connected to the piston has an inclined end face which wobbles upon the rotation of the gear and is pressed by a helical spring against a supporting ball which is positioned in a recess of a flange bolt manually turnable from the outside of the housing. The path of adjustment of the supporting ball is greater than the diameter of the piston. The working pressure generated by the spring is translated directly to the piston.