An organic light-emitting device (OLED) includes a first electrode, a second electrode, an emission layer between the first electrode and the second electrode and including an electron-transporting host and a hole-transporting host, a hole transport region between the first electrode and the emission layer and including a hole transport layer, and an electron transport region between the emission layer and the second electrode and including an electron transport layer, wherein the OLED satisfies Equations 1 and 2 below: 0.75 eV≤|LUMOH(ET)−LUMOH(HT)|≤0.90 eV   |E(S1,H(ET))−E(S1,H(HT))| wherein in Equations 1 and 2, LUMOH(ET) refers to a lowest unoccupied molecular orbital (LUMO) energy level of the electron-transporting host, LUMOH(HT) refers to an LUMO energy level of the hole-transporting host, E(S1, H(ET)) refers to a singlet energy level of the electron-transporting host, and E(S1, H(HT)) refers to a singlet energy level of the hole-transporting host.