An organic light-emitting device includes: a first electrode, a second electrode facing the first electrode, and an organic layer between the first electrode and the second electrode, the organic layer including: an emission layer, an electron transport region between the second electrode and the emission layer, and a mixed layer between the emission layer and the electron transport region, the mixed layer including a first material and a second material, the first material and the second material being selected from a pyrrolidine-based compound and a C10-C30 polycyclicaromatic hydrocarbon-based compound, and a triplet energy EgT1 of at least one selected from the first material and the second material being 2.2 eV or greater.