Arrangement, comprising - a press fitting with at least one connecting piece for mounting a pipe, - plastic parts - a press sleeve, which is secured, opposite an opening of the connecting piece, to the connecting piece via the plastic parts, and - a pressing tool with pressing jaws, which internally comprise multiple ribs (15a to 15c), which can radially encompass the press sleeve, wherein - the connecting piece comprises a circumferential groove with an O-ring received therein, - the outer diameter D - 2Δ of the O-ring is smaller than the largest outer diameter D - Δ of the part of the connecting piece, on which the pipe can be mounted, so that in the non-pressed state, no sealing between the O-ring and the pipe to be mounted can be achieved, - the connecting piece is configured to be pressed using the pressing tool with the multiple ribs (15a to 15c) spaced in the longitudinal direction of the connecting piece, characterized in that the arrangement is formed in such a manner that, during pressing, a middle rib of the pressing tool is positioned in alignment relative to the O-ring, and the other ribs (15a, 15c) are arranged each in alignment relative to displacement spaces (7a, 7b) formed circumferentially on the connecting piece in the region of the other ribs (15a, 15c), which displacement spaces are arranged in the vicinity of the opening of the connecting piece and in the vicinity of an opposite end of the connecting piece, wherein the displacement spaces (7a, 7b) are configured, during pressing, to partially accommodate the material of the pipe partially displaced through the other ribs (15a, 15c). - a press fitting with at least one connecting piece for mounting a pipe, - plastic parts - a press sleeve, which is secured, opposite an opening of the connecting piece, to the connecting piece via the plastic parts, and - a pressing tool with pressing jaws, which internally comprise multiple ribs (15a to 15c), which can radially encompass the press sleeve, wherein - the connecting piece comprises a circumferential groove with an O-ring received therein, - the outer diameter D - 2Δ of the O-ring is smaller than the largest outer diameter D - Δ of the part of the connecting piece, on which the pipe can be mounted, so that in the non-pressed state, no sealing between the O-ring and the pipe to be mounted can be achieved, - the connecting piece is configured to be pressed using the pressing tool with the multiple ribs (15a to 15c) spaced in the longitudinal direction of the connecting piece, characterized in that the arrangement is formed in such a manner that, during pressing, a middle rib of the pressing tool is positioned in alignment relative to the O-ring, and the other ribs (15a, 15c) are arranged each in alignment relative to displacement spaces (7a, 7b) formed circumferentially on the connecting piece in the region of the other ribs (15a, 15c), which displacement spaces are arranged in the vicinity of the opening of the connecting piece and in the vicinity of an opposite end of the connecting piece, wherein the displacement spaces (7a, 7b) are configured, during pressing, to partially accommodate the material of the pipe partially displaced through the other ribs (15a, 15c).