Abstract: A motor, expander, compressor, or hydraulic device is formed with an oscillating rotating piston comprising a cylinder having an axis of rotation and end surfaces and defining an oscillating compression volume and an expansion volume. An axial sealing member separates the compression volume and the expansion volume, and seal members seal the end surfaces of the piston. Valves operate to close the compression volume and open the expansion volume at each oscillation of the piston. Means are provided for reversing the rotation of the cylinder at the end of a compression cycle of the piston. One or more pistons may be provided that contact other pistons along axial surfaces to form axial seal surfaces with rolling contacts that reduce friction energy losses.