Oscillation system of a swingarm of a back wheel with respect to the frame of a two-wheeled vehicle, wherein said swingarm is rotatably constrainable to said frame at a first pin, said system comprising at least one connecting rod and at least one crank, wherein said connecting rod comprises a first end connectable to said frame and a second end rotatably constrained to said crank at a further first end of said crank, and wherein said crank comprises a further second end rotatably constrained to said swingarm at a second pin distinct from said first pin, wherein said connecting rod and/or said crank comprise(s) means to vary the distance in a static and/or dynamic manner between said first end and said second end of said connecting rod, respectively, and/or between said further first end and said further second end of said crank, said distance variation means comprising at least one fluid-transfer device, said fluid-transfer device comprising a cylinder having a first chamber and a second chamber, and a piston sliding within said cylinder thus varying the dimensions of said first chamber and said second chamber, and wherein said cylinder is directly or indirectly integral with a first portion of said connecting rod, or of said crank, and said piston is directly or indirectly integral with a second portion of said connecting rod, or of said crank, and wherein said distance variation means comprise a gas spring functionally combined with said second chamber and/or with said first chamber of said cylinder, wherein said gas spring comprises a third chamber and a fourth chamber, said third chamber containing a gas, wherein said third chamber and said fourth chamber are separated from each other, directly or indirectly, by at least one dividing wall which is sliding, or deformable, in such a way to vary the volume of said third chamber and said fourth chamber depending on the position of said piston with respect to said cylinder to vary the volume, or pressure, of the gas within said third chamber, wherein said fourth chamber is in fluidic communication with said second chamber, said fourth chamber and said second chamber containing an incompressible fluid, said incompressible fluid being transferred between said second chamber and said fourth chamber and vice versa, as the position of said piston with respect to said cylinder varies, said at least one dividing wall also being sliding or deformable depending on the volume of incompressible fluid transferred from said second chamber to said fourth chamber and vice versa, wherein said transfer device comprises at least one duct for fluidically connecting said fourth chamber to said second chamber for transferring said incompressible fluid between said second chamber and said fourth chamber, and vice versa, said transfer device comprising a rod integrally combined with said piston and adapted to slide said piston within said cylinder, said oscillation system being characterized in that said rod is provided with an inner housing comprising a hole formed within said rod, said inner housing comprising said third chamber, said fourth chamber and said dividing wall, said duct being made at least partially within said rod and/or said piston to fluidically connect said second chamber to said fourth chamber, and in that said gas spring comprises first adjustment means for adjusting the pressure of said gas within said third chamber, said gas spring comprising a fifth chamber and at least one second dividing wall to separate said third chamber from said fifth chamber, said second dividing wall being sliding or deformable, so as to vary the volume of said third chamber and said fifth chamber, said fifth chamber and said second dividing wall being arranged within said hole, said first adjustment means also comprising a sixth chamber fluidically connected to said fifth chamber, said sixth chamber and said fifth chamber containing an incompressible fluid, said incompressible fluid being transferred by said first adjustment means between said fifth chamber and said sixth chamber and vice versa, said at least one second dividing wall being further sliding or deformable, depending on the volume of incompressible fluid transferred from said fifth chamber to said sixth chamber, and vice versa.