We show how to efficiently calculate the signal in optical coherence tomography (OCT) systems due to the ballistic photons, the quasi-ballistic photons, and the photons that undergo multiple diffusive scattering using Monte Carlo simulations with importance sampling. This method enables the calculation of these three components of the OCT signal with less than one hundredth of the computational time required by the conventional Monte Carlo method. Therefore, it can be used as a design tool to characterize the performance of OCT systems, and can also be used in the development of novel signal processing techniques that can extend the imaging range of OCT systems. We investigate the parameter dependence of our importance sampling method and we validate it by comparison to an existing method. © 2012 Optical Society of America.