Ar atmospheric pressure plasma (APP) was used to treat indium-tin-oxide (ITO). The plasma conditions were varied to treat the ITO surface, e.g., plasma treatment time, RF power, flow rate, and the plasma outlet-to-sample distance. The plasma effectiveness was measured by the contact angle. The change in the surface energy calculated with the Owens–Wendt method mainly arises from the polar component. The dynamic contact angle measurements show that APP-treated surface showed considerably lower hysteresis in the water and ethylene glycol but there was no change in hysteresis in methylene iodide compared with the untreated ITO. Atomic force microscopy showed that the Ar APP-treated surface sharply decreased the surface roughness and showed a similar morphology as the untreated ITO. X-ray photoelectron spectroscopy showed that the Ar APP treatment not only effectively removed carbon contamination from the surface but also introduced oxygen. Therefore, it is believed that the APP treatment modifies the physico-chemical properties of ITO, which can in turn improve the performance of the organic light-emitting diodes.