We investigate the charge injection efficiency of plasma treated indium. tin oxide (ITO) anodes into copper phthalocyanine (CuPc) in single-layer diodes fabricated under inert conditions. Using electroabsorption and Kelvin p'robe surface potential measurements, we demonstrate that the effective ITO work function is pinned at the energy level of the highest occupied molecular orbital of CuPc. We ascribe this effect to oxygen doping from the ITO electrode. Such doping results in high-efficiency hole injection from ITO as inferred from the current-voltage characteristics. We find evidence for a time-dependent modification of the device characteristics particularly in reverse bias that we attribute to oxygen diffusion from ITO into bulk CuPc. Oxygen plasma treatment of ITO produces an oxide surface that is stable with respect to oxygen diffusion.