Significance We show that a second-phase diagram is needed to describe “overdoped” cuprates prepared by high-pressure oxygen methods. The transition temperatures of HPO compounds continue to increase past the values of O stoichiometry and Cu charge where the superconductivity in conventional materials terminates. The Mo and Cu maintain their preferred geometries despite the higher Cu charge via clustering of the Mo. The principal effect of the Mo and excess O is therefore nanophase separation into Mo- and Cu-enriched domains. The high degree of similarity between the HPO and normal structures, including a Cu2-apical O two-site distribution, therefore suggests that the elevated T _c of YSCO–Mo may originate in a combination of this nanoscale heterogeneity and the dynamical properties.