In order to elucidate the unusual superconducting properties of cuprates in the strongly overdoped region, i.e., at hole-doping levels (Formula presented.) /Cu in the CuO (Formula presented.) plane, we study the structural and superconducting properties of a series of Cu (Formula presented.) Mo (Formula presented.) Sr (Formula presented.) YCu (Formula presented.) O (Formula presented.) powder samples oxygenated under high pressure using different concentrations of KClO (Formula presented.) up to 35 mol %. The analysis of X-ray diffraction data indicates a high purity ∼90% of all samples and suggests that the concentration, x, of extra oxygen atoms increases with increasing KClO (Formula presented.) concentration. Surprisingly, the (Formula presented.) values remain nearly constant within the 80–85 K range independent of KClO (Formula presented.) concentration, which suggests a scenario of (Formula presented.) saturation. In order to account for this unexpected behaviour, we put forward the hypothesis that overdoping enhances the density of unpaired holes, which is supported by the observation of large values of the Sommerfeld coefficient in all samples. We therefore propose a scenario of electronic phase separation between normal and superconducting holes.