Oscillation mechanism for a chair or an armchair

An oscillation mechanism for a chair or an armchair comprising a seat and a backrest, wherein the oscillation mechanism comprises:

• a functional body hingeable to the backrest in a rear hinge axis and hingeable to the seat in a front hinge axis, wherein the functional body comprises at least one flexible functional portion elastically bendable between the front hinge axis and the rear hinge axis for an oscillation of the seat and/or the backrest;
• and
• an adjustment device for adjusting a flexibility of the functional body, comprising:
• a support portion connected to the at least one flexible functional portion;
• at least one auxiliary portion comprising a first end connected to the at least one flexible functional portion, and a free end;
• and
• a movable engagement member supported by the support portion;
• wherein the adjustment device is configurable in:
• a high-flexibility configuration in which the movable engagement member is disengaged from the free end of the auxiliary portion, so that the auxiliary portion is configured in a cantilever fashion;
• and
• a reduced flexibility configuration in which the movable engagement member engages the free end, so that the auxiliary portion elastically bends between the front hinge axis and the rear hinge axis together with the at least one flexible functional portion.