A composition including a platinum-containing organometallic compound, a first compound, a second compound, and a third compound, and an organic light-emitting device including the same wherein the composition does not comprise iridium, the Pt-containing organometallic compound, the first compound, the second compound, and the third compound are different from each other, the first compound comprises at least one electron transport moiety, the second compound and the third compound do not include a metal, each of an absolute value of a HOMO energy level of the second compound and an absolute value of a HOMO energy level of the third compound is 5.30 eV to 5.85 eV, the difference between the absolute value of the HOMO energy level of the second compound and the absolute value of the HOMO energy level of the third compound is 0.01 eV to 0.30 eV.