An oscillating system for a mechanical timepiece includes an annular balance wheel arranged in a fixed manner coaxially in relation to a rotatably mounted balance staff. A helical spring encloses the balance staff and has its inner end fastened on the balance staff and its outer end fastened on a fastening device. The fastening device has a helical-spring connector with an inner clamping jaw and an outer clamping jaw, the inner clamping jaw being radially inside of the outer clamping jaw in relation to the axis of rotation of the balance staff. The outer end of the helical spring may be clamped firmly between a clamping surface of the inner clamping jaw and a clamping surface of the outer clamping jaw.
