Embodiments of the present disclosure provide an apparatus and method for mounting an O-ring to a work piece. The apparatus comprises a guiding component comprising a frustum portion adapted to be arranged coaxially with a groove formed on an end surface of the work piece for receiving the O-ring, a large end face of the frustum portion arranged adjacent to the end surface; a plurality of through slots each extending a predetermined distance radially from an edge of the large end face; and a pressing component comprising a plurality of fingers each adapted to be fitted in and moved along the corresponding through slot to push the O-ring that is placed on the frustum portion into the groove, wherein a diameter of the large end face is larger than an inner diameter of the groove.