An organometallic compound represented by Formula 1A or Formula 1C: wherein, in Formula 1A or 1C: M is a transition metal; X 1 is N or C(R 5); R 1, R 4 and R 5 are each independently selected from the group of a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine, a hydrazone, a carboxyl group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a substituted or unsubstituted C 1 -C 60 alkyl group, a substituted or unsubstituted C 2 -C 60 alkenyl group, a substituted or unsubstituted C 2 -C 60 alkynyl group, a substituted or unsubstituted C 1 -C 60 alkoxy group, a substituted or unsubstituted C 3 -C 10 cycloalkyl group, a substituted or unsubstituted C 3 -C 10 cycloalkenyl group, a substituted or unsubstituted C 3 -C 10 heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10 heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60 aryl group, a substituted or unsubstituted C 6 -C 60 aryloxy group, a substituted or unsubstituted C 6 -C 60 arylthio group, a substituted or unsubstituted C 2 -C 60 heteroaryl group, -N(Q 1)(Q 2), -Si(Q 3)(Q 4)(Q 5), -C(=O)(Q 6), and a binding site with an adjacent ligand via a single bond or a divalent linking group; the A ring and the B ring are each independently selected from the group of a substituted or unsubstituted C 4 -C 20 alicyclic ring, a substituted or unsubstituted C 2 -C 20 heteroalicyclic ring, a substituted or unsubstituted C 6 -C 20 aromatic ring, and a substituted or unsubstituted C 2 -C 20 heteroaromatic ring; Q 1 to Q 6 are each independently selected from the group of a hydrogen atom, a substituted or unsubstituted C 1 -C 60 alkyl group, a substituted or unsubstituted C 6 -C 60 aryl group, and a substituted or unsubstituted C 2 -C 60 heteroaryl group; n is an integer from 1 to 3; L is a monodentate, bidentate, tridentate, or tetradentate organic ligand; and m is an integer from 0 to 4. M is a transition metal; X 1 is N or C(R 5); R 1, R 4 and R 5 are each independently selected from the group of a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine, a hydrazone, a carboxyl group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a substituted or unsubstituted C 1 -C 60 alkyl group, a substituted or unsubstituted C 2 -C 60 alkenyl group, a substituted or unsubstituted C 2 -C 60 alkynyl group, a substituted or unsubstituted C 1 -C 60 alkoxy group, a substituted or unsubstituted C 3 -C 10 cycloalkyl group, a substituted or unsubstituted C 3 -C 10 cycloalkenyl group, a substituted or unsubstituted C 3 -C 10 heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10 heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60 aryl group, a substituted or unsubstituted C 6 -C 60 aryloxy group, a substituted or unsubstituted C 6 -C 60 arylthio group, a substituted or unsubstituted C 2 -C 60 heteroaryl group, -N(Q 1)(Q 2), -Si(Q 3)(Q 4)(Q 5), -C(=O)(Q 6), and a binding site with an adjacent ligand via a single bond or a divalent linking group; the A ring and the B ring are each independently selected from the group of a substituted or unsubstituted C 4 -C 20 alicyclic ring, a substituted or unsubstituted C 2 -C 20 heteroalicyclic ring, a substituted or unsubstituted C 6 -C 20 aromatic ring, and a substituted or unsubstituted C 2 -C 20 heteroaromatic ring; Q 1 to Q 6 are each independently selected from the group of a hydrogen atom, a substituted or unsubstituted C 1 -C 60 alkyl group, a substituted or unsubstituted C 6 -C 60 aryl group, and a substituted or unsubstituted C 2 -C 60 heteroaryl group; n is an integer from 1 to 3; L is a monodentate, bidentate, tridentate, or tetradentate organic ligand; and m is an integer from 0 to 4.