We have scrutinized the charge injection efficiency from plasma treated indium tin oxide anodes into CuPc using single layer diodes fabricated under inert conditions. Current-voltage characteristics demonstrate that the hole injection from ITO into CuPc is identical for all devices, regardless of the oxide electrode surface treatment. When the devices are aged for several months in a glovebox, the current-voltage characteristics in the reverse differ in a striking way. We demonstrate that these characteristics are due to the interplay between two features, the formation of an oxygen enriched interface layer at the electrode and the doping of the CuPc film due to diffusion of oxygen out of the oxide electrode. By electro-absorption measurements and Kelvin Probe surface potential measurements we obtain a coherent picture of the most influent factors determining the characteristics of freshly fabricated and aged devices. As an important conclusion, we find that the oxygen plasma treatment of the indium tin oxide electrode produces a much more stable surface with regards to oxygen diffusion.