An organic light-emitting device including a first electrode; a second electrode facing the first electrode; an organic layer between the first electrode and the second electrode, the organic layer including an emission layer, and a light-efficiency improvement layer that includes at least one selected from a first light-efficiency improvement layer and a second light-efficiency improvement layer, wherein the first light-efficiency improvement layer is disposed in a path of light that is generated in the emission layer and emitted toward an outside of the organic light-emitting device through the first electrode; the second light-efficiency improvement layer is disposed in a path of light that is generated in the emission layer and emitted toward the outside of the organic light-emitting device through the second electrode; and the light-efficiency improvement layer includes a phosphine oxide compound represented by the following Formula 1: