A method of installing rotor blades on an offshore wind turbine includes arranging a tower of the wind turbine at an offshore installation site. Then a first end of a tensioner is fastened to at least one of the tower, the nacelle, and the hub at a distance above sea level. A second end of the tensioner is fastened to a holding device floating in the sea at a distance from the tower. Then a pulling force is applied to the tensioner from the holding device at least part of the time while the rotor blades are being mounted to the hub, so that oscillations of the tower due to external forces are dampened during installation of the rotor blades.