We has been analysed optical properties of OLED(organic light emitting diode) and characteristics of ITO(Indium Tin Oxide) in terms of O2 plasma treatment for manufacturing high efficiency OLED. RF power of O2 plasma was changed 25, 50, 100, 200 W. O2 gas flow, gas pressure and treatment time were fixed. Sheet resistance and surface roughness of ITO were measured by Hall-effect measurement system and AFM, respectively. The ranges of sheet resistance and surface roughness were 5.5～6.06 Ω and 2.438～3.506 nm changing of RF power, respectively. PM(Passive Matrix)OLED was fabricated with the structure of ITO(plasm treatment)/TPD(400 Å)/Alq3(600 Å)/LiF(5 Å)/Al(1200 Å). Turn-on voltage of PMOLED was 7 V and luminance was 7,371 cd/m2 at the RF power of 25 W. O2 plasma treatment of ITO surface was result in lowering the operating voltage and improving luminance of PMOLED.