An organic light-emitting device includes: a first electrode; a second electrode facing the first electrode; an emission layer between the first electrode and the second electrode; a hole transport region between the first electrode and the emission layer; and an electron transport region between the emission layer and the second electrode, the electron transport region including an electron auxiliary layer, wherein the hole transport region includes at least one copolymer selected from i) a first copolymer of a first compound and a second compound and ii) a second copolymer of a third compound and a fourth compound, wherein the first compound and the second compound are each independently selected from compounds represented by Formula 1, and the third compound and the fourth compound are each independently selected from compounds represented by Formula 2, and wherein the electron auxiliary layer includes a metal oxide.