Optically transparent indium-tin oxide (ITO) is a "universal" electrode for various optoelectronic devices such as organic light emitting diodes (OLEDs). It is known that the performance of OLEDs improves significantly by exposing the ITO surface to an oxygen plasma. This study employs conducting atomic force microscopy (C-AFM) for unique nanometer-scale mapping of the local current density of a vapor-deposited ITO film. The local conductance is shown to increase by orders of magnitude and becomes more uniform after oxygen plasma treatment for measurements of the identical 200-nm(2) regions. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy measurements of separate regions of the same films suggest that the oxygen plasma removes a thin layer of insulating carbon-rich material from the surface. The extensive heterogeneity in interfacial electrical conductivity measured by C-AFM calls into question previous studies of STM-induced electroluminescence: of polymer films on ITO as well as STM imaging of such films. The impact of this study on the future development of optoelectronic devices is discussed.