An organometallic compound represented by by one of Formulae 1-1 to 1-3: wherein, in Formulae 1-1 to 1-3, Y 1 to Y 3 are each independently a divalent linking group; M is selected from a Period 1 transition metal, a Period 2 transition metal, and a Period 3 transition metal, preferably selected from iridium (Ir), platinum (Pt), osmium (Os), ruthenium (Ru), rhodium (Rh), palladium (Pd), copper (Cu), silver (Ag), gold (Au), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), and thulium (Tm) and more preferably selected from Ir, Pt, and Os; A 1 to A 4 are each independently selected from a benzene group, a naphthalene group, a pyridine group, a pyrimidine group, a pyrazine group, a quinoline group, an isoquinoline group, a dibenzofuran group, and a dibenzothiophene group; and at least one selected from A 3 and A 4 is selected from a pyridine group, a pyrimidine group, a pyrazine group, a quinoline group, and an isoquinoline group. X 1 to X 4 are each independently selected from a carbon atom and a nitrogen atom, provided that at least one selected from X 3 and X 4 is N; Z 1 and Z 2 are each independently represented by one of Formulae 2-21 to 2-34: wherein, in Formulae 2-21 to 2-34, Et refers to an ethyl group; Ph refers to a phenyl group; and * indicates a binding site to a neighboring atom; d1 and d2 are each independently selected from 0, 1, 2, 3, and 4; when d1 is 2 or more, groups Z 1 are identical to or different from each other; when d2 is 2 or more, groups Z 2 are identical to or different from each other; when X 3 is N, d1 is selected from 1, 2, 3, and 4; or when X 4 is N, d2 is selected from 1, 2, 3, and 4; R 1 to R 4 are each independently selected from hydrogen, deuterium, a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an iso-pentyl group, a sec-pentyl group, and a tert-pentyl group; a phenyl group and a carbazolyl group; and a phenyl group, a naphthyl group, and a carbazolyl group, each substituted with at least one selected from a C 1 -C 20 alkyl group and a phenyl group; b1 to b4 are each independently selected from 1, 2, 3, and 4; L 1 is selected from a monodentate ligand and a bidentate ligand; a1 is selected from 0, 1, and 2; and * indicates a binding site to a neighboring atom. Y 1 to Y 3 are each independently a divalent linking group; M is selected from a Period 1 transition metal, a Period 2 transition metal, and a Period 3 transition metal, preferably selected from iridium (Ir), platinum (Pt), osmium (Os), ruthenium (Ru), rhodium (Rh), palladium (Pd), copper (Cu), silver (Ag), gold (Au), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), and thulium (Tm) and more preferably selected from Ir, Pt, and Os; A 1 to A 4 are each independently selected from a benzene group, a naphthalene group, a pyridine group, a pyrimidine group, a pyrazine group, a quinoline group, an isoquinoline group, a dibenzofuran group, and a dibenzothiophene group; and at least one selected from A 3 and A 4 is selected from a pyridine group, a pyrimidine group, a pyrazine group, a quinoline group, and an isoquinoline group. X 1 to X 4 are each independently selected from a carbon atom and a nitrogen atom, provided that at least one selected from X 3 and X 4 is N; Z 1 and Z 2 are each independently represented by one of Formulae 2-21 to 2-34: Et refers to an ethyl group; Ph refers to a phenyl group; and * indicates a binding site to a neighboring atom; d1 and d2 are each independently selected from 0, 1, 2, 3, and 4; when d1 is 2 or more, groups Z 1 are identical to or different from each other; when d2 is 2 or more, groups Z 2 are identical to or different from each other; when X 3 is N, d1 is selected from 1, 2, 3, and 4; or when X 4 is N, d2 is selected from 1, 2, 3, and 4; R 1 to R 4 are each independently selected from hydrogen, deuterium, a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an iso-pentyl group, a sec-pentyl group, and a tert-pentyl group; a phenyl group and a carbazolyl group; and a phenyl group, a naphthyl group, and a carbazolyl group, each substituted with at least one selected from a C 1 -C 20 alkyl group and a phenyl group; b1 to b4 are each independently selected from 1, 2, 3, and 4; L 1 is selected from a monodentate ligand and a bidentate ligand; a1 is selected from 0, 1, and 2; and * indicates a binding site to a neighboring atom. when d1 is 2 or more, groups Z 1 are identical to or different from each other; when d2 is 2 or more, groups Z 2 are identical to or different from each other; when X 3 is N, d1 is selected from 1, 2, 3, and 4; or when X 4 is N, d2 is selected from 1, 2, 3, and 4; R 1 to R 4 are each independently selected from hydrogen, deuterium, a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an iso-pentyl group, a sec-pentyl group, and a tert-pentyl group; a phenyl group and a carbazolyl group; and a phenyl group, a naphthyl group, and a carbazolyl group, each substituted with at least one selected from a C 1 -C 20 alkyl group and a phenyl group; b1 to b4 are each independently selected from 1, 2, 3, and 4; L 1 is selected from a monodentate ligand and a bidentate ligand; a1 is selected from 0, 1, and 2; and * indicates a binding site to a neighboring atom.