An optical sensor has a body including a transparent interface wall having an outer surface in contact with a liquid, to define therewith a reflection interface. An optical module is coupled at an inner surface of the interface wall, to detect a characteristic of the liquid substance via light reflection. The optical module includes a light emitter and a light receiver facing respective inclined surfaces of the interface wall. The optical module also includes a supporting structure which is elastically flexible, to enable the light emitter and the light receiver of the optical module to vary a relative position thereof and with respect to the inclined surfaces of the interface wall, when the supporting structure is secured to the inner surface of the interface wall.