An organic light-emitting device (OLED) is disclosed. The OLED includes a light-emitting layer, a first electrode, and a second electrode, in which the light-emitting layer is interposed between the first and the second electrodes and includes a first molecular energy level of a host, and a second molecular energy level of a dopant. The first molecular energy level has a highest occupied molecular orbital (HOMO) which is substantially same as the HOMO of the second molecular energy level, or the first molecular energy level has a lowest unoccupied molecular orbital (LUMO) which is substantially the same as the LUMO of the second molecular energy level.