In this paper, we demonstrate the numerical simulations of the organic light-emitting device (OLED) based on a rigorous and efficient numerical method. The input parameters of such a program include the layer thickness and complex refractive index of each layer, the locations and density of the oscillating dipoles, and the emission photoluminescence spectrum. In evaluating the device performances, the output spectrum, the intensity distribution, and the viewing-angle characteristics of an OLED are concerned. Since the numerical difficulty arising from the large thickness of the glass layer in the OLED is carefully overcome, the program can simulate the optical performances with different glass substrate thickness ranged from less than one to hundreds of μm. When the glass substrate becomes thinner, multi-peak spectrum of an OLED is observed due to the strong interference effect between the two sides of the glass substrate. When the thickness of the glass substrate is reduced to less than 1 μm, the device is identical to a top-emission OLED with a two-microcavity structure. The simulation results are consistent with the Fabry-Perot cavity equation, which can be used as a guideline for optimizing the optical characteristics of an OLED from the normal direction. We have also demonstrated the procedures to maximize the total flux from an OLED which is more important than the luminance from the normal direction for the lighting application. Since our development of numerical algorithms is based on the general electromagnetic theory, the proposed model is, in principle, applicable to an OLED consisting of any number of layers.