The invention provides heterocyclic compounds, an organic light-emitting device comprising the heterocyclic compounds and an organic light-emitting display comprising the organic light-emitting device. The heterocyclic compounds may comprise an aromatic ring or a heteroaromatic ring fused with a carbazole, dibenzothiophene, or dibenzofurane derivative, the compounds featuring rigid backbone structures with high glass transition temperatures and high melting points. When the heterocyclic compounds are applied to the organic light-emitting device, the organic light-emitting device may exhibit high electrical stability, improved charge transport ability, high heat resistance and improved light-emitting properties. The organic light-emitting display prepared according to the present invention exhibit lower driving voltages, increased luminescent efficiencies and longer lifetimes.