The presence of electrically active impurities in crystalline silicon causes changes in its optical properties which interfere with IR absorbance measurements of interstitial oxygen content. In this paper we report the use of a theoretical model to accurately simulate the baseline IR absorbance spectrum of n-type silicon over a range of resistivities. This information is used to accurately compensate measured spectra for the effects of free carrier absorption. In addition, empirical limits are set on the range of resistivities amenable to oxygen measurements.