An organic light emitting diode (OLED) includes a first emitting material layer disposed adjacently to a first electrode and a second emitting material layer disposed adjacently to a second electrode. The first emitting material layer includes a first compound having high affinity to holes and a second compound of thermally activated delayed fluorescent material, and the second emitting material layer includes a fourth compound having high affinity to electrons and implementing triplet-triplet annihilation. Two emitting material layers having adjusted energy levels allow the OLED to lower its driving voltages and improve its luminous efficiency and luminous lifetime.