The optical surface analysis code (OSAC) is used to predict the optical performance of the Hubble Space Telescope. We describe the analysis in detail and compare some of the results with those of a standard optical transfer function analysis. Also, we discuss the mathematical foundation of the OSAC code and describe in detail the error data that were used as input. OSAC is shown to be a valuable tool for separating and analyzing the effects of misalignments, figure errors, and mid and high frequency statistical mirror surface errors on optical performance.