The present application relates to an organic light-emitting device and a flat display apparatus including the same. An organic light-emitting device includes: a first electrode; a second electrode facing the first electrode; light-emitting units in the number of m between the first electrode and the second electrode; and charge generation layers in the number of m−1 respectively between each pair of adjacent light-emitting units among the light-emitting units in the number of m, each charge generation layer including an n-type charge generation layer and a p-type charge generation layer, wherein m is an integer of 3 or more, the light-emitting units in the number of m each include a hole transport region, an emission layer, and an electron transport region, which are sequentially stacked, electron transport regions in the number of m included in the light-emitting units in the number of m each include an electron transport material, and an electron transport material included in at least one electron transport region among the electron transport regions in the number of m is different from an electron transport material included in at least one electron transport region among the other electron transport regions.