An organic light-emitting device and a flat panel display device, the organic-light emitting device including an anode; a cathode; and an organic layer therebetween including an emission layer, a hole transport region between the anode and the emission layer, the hole transport region including at least one of a hole injection layer, a hole transport 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 layers and an electron injection layer, and a buffer layer between the emission layer and the electron transport region, wherein the buffer layer includes a biscarbazole-based derivative and triphenylene-based derivative, and a triplet energy (ET1) of the biscarbazole-based derivative or the triphenylene-based derivative and a triplet energy (ET2) of a dopant of the emission layer satisfy the following relationship: ET1>ET2.