A top-emitting organic light-emitting device (OLED) including a reflective first electrode layer; a semitransparent second electrode layer facing the first electrode layer; and an organic layer between the first electrode layer and the second electrode layer, wherein the organic layer comprises a first layer, a second layer, and an emission layer between the first layer and the second layer, wherein the sum of the thicknesses of the emission layer and the second layer is about 0.15 to about 0.30 times the thickness of the entire organic layer. The top-emitting OLED may have high efficiency, and thus driving voltage may be reduced.