An organic light-emitting device comprising: a first electrode; a second electrode facing the first electrode; and an organic layer between the first electrode and the second electrode and comprising an emission layer, wherein: the emission layer comprises a host and a dopant, the host comprises a first host compound, a second host compound, and a third host compound, and the first host compound satisfies Conditions 1-1 and 1-2: S 1 H 1 > 3 eV T 1 H 1 > 2.8 eV, and wherein, in Conditions 1-1 and 1-2, S1 indicates a lowest excitation singlet energy level of the first host compound, measured by preparing on a quartz substrate a sample having a thickness of 1000 Å and analyzing the obtained absorption spectrum from the sample at room temperature; and T1 indicates a lowest excitation triplet energy level of the first host compound, measured by preparing on a quartz substrate a sample having a thickness of 1000 Å and analyzing the peak with the shortest wavelength obtained from the photoluminescent emission spectrum of the sample at temperature of 77K; wherein the first host compound and the second host compound satisfy Condition 3-1, and the first host compound and the third host compound satisfy Condition 3-2: HOMO H 1 > HOMO H 2 HOMO H 1 > HOMO H 3, wherein, in Conditions 3-1 and 3-2, HOMO indicates a highest occupied molecular orbital (HOMO) energy level of the first host compound, HOMO indicates a HOMO energy level of the second host compound, and HOMO indicates a HOMO energy level of the third host compound. a first electrode; a second electrode facing the first electrode; and an organic layer between the first electrode and the second electrode and comprising an emission layer, wherein: the emission layer comprises a host and a dopant, the host comprises a first host compound, a second host compound, and a third host compound, and the first host compound satisfies Conditions 1-1 and 1-2: S 1 H 1 > 3 eV T 1 H 1 > 2.8 eV, and wherein, in Conditions 1-1 and 1-2, S1 indicates a lowest excitation singlet energy level of the first host compound, measured by preparing on a quartz substrate a sample having a thickness of 1000 Å and analyzing the obtained absorption spectrum from the sample at room temperature; and T1 indicates a lowest excitation triplet energy level of the first host compound, measured by preparing on a quartz substrate a sample having a thickness of 1000 Å and analyzing the peak with the shortest wavelength obtained from the photoluminescent emission spectrum of the sample at temperature of 77K; wherein the first host compound and the second host compound satisfy Condition 3-1, and the first host compound and the third host compound satisfy Condition 3-2: HOMO H 1 > HOMO H 2 HOMO H 1 > HOMO H 3, the emission layer comprises a host and a dopant, the host comprises a first host compound, a second host compound, and a third host compound, and the first host compound satisfies Conditions 1-1 and 1-2: S 1 H 1 > 3 eV T 1 H 1 > 2.8 eV, and wherein, in Conditions 1-1 and 1-2, S1 indicates a lowest excitation singlet energy level of the first host compound, measured by preparing on a quartz substrate a sample having a thickness of 1000 Å and analyzing the obtained absorption spectrum from the sample at room temperature; and T1 indicates a lowest excitation triplet energy level of the first host compound, measured by preparing on a quartz substrate a sample having a thickness of 1000 Å and analyzing the peak with the shortest wavelength obtained from the photoluminescent emission spectrum of the sample at temperature of 77K; wherein the first host compound and the second host compound satisfy Condition 3-1, and the first host compound and the third host compound satisfy Condition 3-2: HOMO H 1 > HOMO H 2 HOMO H 1 > HOMO H 3, wherein, in Conditions 3-1 and 3-2, HOMO indicates a highest occupied molecular orbital (HOMO) energy level of the first host compound, HOMO indicates a HOMO energy level of the second host compound, and HOMO indicates a HOMO energy level of the third host compound.