An organic light-emitting device including 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 material emitting delayed fluorescence, the first host is a compound represented by Formula 1 below, and the second host is a compound represented by any one of Formulae 2-1, 2-2, and 2-3 below: wherein X, X2, X3, Y1 to Y4, L1 to L7, R1 to R11, a, a1, a2, b, b1, b2, c, c1, c2, d, e, f, and g are as defined in the specification.