In an oil-sealed bearing assembly in the bearing section of a mud motor, in which a mandrel is rotatably disposed within a cylindrical housing, a cylindrical sleeve is disposed coaxially and rotatably around the mandrel, with the sleeve being non-rotatably mounted to an inner surface of the housing, thereby forming an annular piston chamber forming part of a generally annular oil reservoir. The sleeve provides radial support to the mandrel along the length of the piston chamber by virtue of the flexural stiffness of the sleeve. The mandrel rotates within the sleeve, with a radial bearing being provided between the sleeve and the mandrel. An annular piston is axially movable within the piston chamber in response to variations in the volume of oil in the reservoir. The piston slides within the piston chamber without rotation relative to either the housing or the sleeve.