An organic light-emitting device includes an organic layer including an emission layer between a first electrode and the second electrode, and a hole transport region including an auxiliary layer between the first electrode and the emission layer, the hole transport region. The auxiliary layer includes a first material and a second material that satisfy Equations 1-1 and 1-2: 0 eV 0 eV wherein in Equation 1-1 and 1-2, EH1 is a highest occupied molecular orbital energy (HOMO energy) of the first material; EL1 is a lowest unoccupied molecular orbital energy (LUMO energy) of the first material; EH2 is a HOMO energy of the second material; and EL2 is a LUMO energy of the second material.