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. The emission layer includes at least one organometallic compound of Formula 1, and at least one selected from a second compound and a third compound, where the organometallic compound, the second compound, and the third compound are different from each other. When the emission layer comprises both the second compound and the third compound, the second compound and the third compound form an exciplex, and the organometallic compound and the second compound and/or the third compound do not form an exciplex: M(L1)n1(L2)n2,  Formula 1 wherein in Formula 1, L1 is represented by Formula 2-1: