Computer simulations of light transport in multi-layered turbid media are an effective way to theoretically investigate light transport in tissue, which can be applied to the analysis, design and optimization of optical coherence tomography (OCT) systems. We present a computationally efficient method to calculate the diffuse reflectance due to ballistic and quasi-ballistic components of photons scattered in turbid media, which represents the signal in optical coherence tomography systems. Our importance sampling based Monte Carlo method enables the calculation of the OCT signal with less than one hundredth of the computational time required by the conventional Monte Carlo method. It also does not produce a systematic bias in the statistical result that is typically observed in existing methods to speed up Monte Carlo simulations of light transport in tissue. This method can be used to assess and optimize the performance of existing OCT systems, and it can also be used to design novel OCT systems. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).