Effects of an Al2O3 nanothin film between the emitting layer and the sputterdeposited cathode were studied for organic light-emitting diodes (OLEDs) with indium–tin–oxide, NN′-dephenyl-NN′-bis)(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine, tris(8-hydroxyquinoline)aluminum (Alq3), and aluminum (Al) as an anode, a hole transport layer, an emitting layer (EML), and a cathode, respectively. The performance of the OLEDs with sputter-deposited Al cathodes was inferior to that of the OLED with the evaporated Al cathode. However, the insertion of an Al2O3 nanothin film with a proper thickness between the EML and the sputter-deposited Al cathode was effective to alleviate performance degradation of the OLEDs with sputter-deposited Al cathodes in current flow and light emission. It is considered that both protection of EML by Al2O3 from the sputtering damage and higher luminance by the presence of a thin insulating layer between the EML and the cathode alleviated performance degradation of the OLED with an Al2O3 cathode buffer layer. The Al2O3 buffer layer could not alleviate quantum efficiency reduction caused by the sputtering damage.