The recent derivation of a lower limit for the N-14/N-15 ratio in Saturn's ammonia, which is found to be consistent with the Jovian value, prompted us to revise models of Saturn's formation using as constraints the supersolar abundances of heavy elements measured in its atmosphere. Here we find that it is possible to account for both Saturn's chemical and isotopic compositions if one assumes the formation of its building blocks at similar to 45 K in the protosolar nebula, provided that the O abundance was similar to 2.6 times protosolar in its feeding zone. To do so, we used a statistical thermodynamic model to investigate the composition of the clathrate phase that formed during the cooling of the protosolar nebula and from which the building blocks of Saturn were agglomerated. We find that Saturn's O/H is at least similar to 34.9 times protosolar and that the corresponding mass of heavy elements (similar to 43.1 M-circle dot) is within the range predicted by semi-convective interior models.