An organic light-emitting device including: a first electrode; a second electrode; an emission layer between the first electrode and the second electrode; and a hole transport region between the first electrode and the emission layer, wherein the hole transport region comprises a first compound represented by Formula 1 and a second compound represented by Formula 2: When the first compound represented by Formula 1 and the second compound represented by Formula 2 are used in the hole transport layer and the hole auxiliary layer, respectively, an organic light-emitting device may have improved driving voltage, efficiency, brightness, and lifespan characteristics.