An organic light-emitting device having improved efficiency and lifespan includes: a first electrode, a second electrode, and an organic layer between the first electrode and the second electrode, wherein the organic layer includes an emission layer, the emission layer includes a first compound, a second compound, a third compound, and a fourth compound, the first compound, the second compound, the third compound, and the fourth compound are different from each other, the third compound includes a metal element having an atomic number of 40 or more, the fourth compound includes boron (B), the third compound and the fourth compound each satisfy Conditions 1-1 and 1-2 below, and the fourth compound satisfies Condition 2 or 3: T1(C3)onset≥S1(C4)onset  Condition 1-1 T1(C3)max≥S1(C4)max  Condition 1-2 KRISC(C4)≥103S−1  Condition 2 f(C4)≥0.1.  Condition 3