An organic light emitting device and a method of fabricating the same are disclosed. The organic light emitting device includes an anode and a cathode that comprises a magnesium-calcium layer. An organic layer having at least an organic emission layer is interposed between the anode and the cathode. The organic light emitting device may have reduced driving voltage characteristics, increased luminous efficiency characteristics, and improved lifespan characteristics without forming a separate electron injection layer because of the excellent electron injection characteristics.