A heterocyclic compound represented by one of Formulae 1-1 to 1-4: wherein, in Formulae 1-1 to 1-4, a1 is 0, and a2 to a3 are each 1, L 2 and L 3 are each independently selected from groups represented by Formulae 4-1 to 4-11: R 1 is selected from groups represented by Formulae 6-1 to 6-12, and R 2 and R 3 are each independently selected from: hydrogen, deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group, a C 1 -C 20 alkyl group substituted with at least one selected from deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group and a C 1 -C 20 alkoxy group substituted with at least one selected from deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group, wherein Formulae 6-1 to 6-12 are: wherein, in Formulae 6-1 to 6-12, * indicates a binding site to a neighboring atom; bi is 1 and b2 and b3 are each independently 1, 2, 3, 4, 5, 6, or 7, when b2 is two or more, two or more R 2 (s) are identical to or different from each other, and when b3 is two or more, two or more R 3 (s) are identical to or different from each other. a1 is 0, and a2 to a3 are each 1, L 2 and L 3 are each independently selected from groups represented by Formulae 4-1 to 4-11: R 1 is selected from groups represented by Formulae 6-1 to 6-12, and R 2 and R 3 are each independently selected from: hydrogen, deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group, a C 1 -C 20 alkyl group substituted with at least one selected from deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group and a C 1 -C 20 alkoxy group substituted with at least one selected from deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group, wherein Formulae 6-1 to 6-12 are: wherein, in Formulae 6-1 to 6-12, * indicates a binding site to a neighboring atom; bi is 1 and b2 and b3 are each independently 1, 2, 3, 4, 5, 6, or 7, when b2 is two or more, two or more R 2 (s) are identical to or different from each other, and when b3 is two or more, two or more R 3 (s) are identical to or different from each other. hydrogen, deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group, a C 1 -C 20 alkyl group substituted with at least one selected from deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group and a C 1 -C 20 alkoxy group substituted with at least one selected from deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20 alkyl group, and a C 1 -C 20 alkoxy group, wherein Formulae 6-1 to 6-12 are: wherein, in Formulae 6-1 to 6-12, * indicates a binding site to a neighboring atom; bi is 1 and b2 and b3 are each independently 1, 2, 3, 4, 5, 6, or 7, when b2 is two or more, two or more R 2 (s) are identical to or different from each other, and when b3 is two or more, two or more R 3 (s) are identical to or different from each other. <EOS>