An organic light-emitting device includes a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode, the emission layer including a dopant, a first host, and a second host. The dopant is a delayed fluorescence emitting material, and a triplet energy of the first host, EH1(T1) and a triplet energy of the second host, EH2(T1) are each equal to or greater than a triplet energy of the dopant, ED(T1). The triplet energy of the first host, EH1(T1) is in a range of about 2.6 eV to about 3.1 eV.