With the advent of renewable energy, stability of power system is becoming more and more crucial. Recently, grid-forming converter concept has attracted much attention from the industry and academia as a potential solution to improve power system stability. This paper proposes a frequency-selective power oscillation damper for grid-forming power converter to improve its flexibility and performance in damping electromechanical modes. The proposed controller employs the multi-rotor concept where a virtual synchronous machine with multiple rotors is emulated in the control part of a grid-forming power converter. In addition to the enhancement in suppressing power oscillation, the main advantages of the proposed controller include the ability to provide the damping selectively for each electromechanical mode. That is, the proposed controller allows setting the desired damping individually for each oscillation mode. The performance of the proposed controller is validated by using the three-machine infinite bus system.