An oscillation mechanism for chairs includes a support frame, a structure coupled to the support frame to rotate about a first axis, an elastic element interposed between the support frame and the structure and configured to oppose a reaction force to an oscillation of the structure. The reaction force generates a reaction moment acting on the structure with respect to the first axis. An oscillation adjustment system is able to vary the reaction moment with the oscillation being unchanged. The adjustment system includes a pin movably coupled to the structure and a movement system of the pin for varying the distance between the pin and the first axis. The elastic element is directly fastened to the pin so that the reaction force is transmitted to the structure with the application point corresponding to the pin and with direction and orientation unchanged.