An organic light-emitting device and a flat panel display device, the organic light-emitting device including an anode; cathode; and organic layer therebetween, the organic layer including a hole transport region between the anode and the emission layer and that includes at least one of a hole injection layer, a hole transport layer, a buffer layer, and an electron blocking layer, an electron transport region between the emission layer and the cathode, the electron transport region including at least one of a hole blocking layer, an electron transport layer, and an electron injection layer, and a mixed organic layer disposed between the emission layer and the electron transport region, wherein the mixed organic layer includes a hole transport compound and an electron transport compound, and an electron affinity (EA1) of the hole transport compound and an electron affinity (EA2) of the electron transport compound satisfy the following relationship: EA1