An organic light-emitting device including an anode electrode, a hole injection layer on the anode electrode, a hole transport layer on the hole injection layer, an emissive layer on the hole transport layer, and a cathode electrode on the emissive layer. A material of the hole injection layer includes a nitrogen-containing compound having a quinoid structure and a nitrogen-containing compound having a benzenoid structure. A ratio of a peak intensity IB to a peak intensity IA (IB/IA) in a Fourier transform infrared spectroscopy (FTIR) spectrum of the material of the hole injection layer ranges from 1.5 to 2.5, the peak intensity IA and the peak intensity IB being further defined.