Accurate evaluation methods for interstitial oxygen concentrations ([O(i)]) by infrared absorptiometry at 1107 cm-1 are investigated for medium resistivity Si crystals with carrier concentrations betwee 3 x 10(15) and 5 x 10(16) cm-3, which correspond to resistivities between 5 and 0.5 OMEGA-cm for p-type crystals. Difficulty of accurate [O(i)] measurements for these crystals originates from the strong dependence of the internal multiple reflection effect inside the sample crystal on the free-carrier absorption intensity. Neglect of this effect introduces an error as large as 1 ppma for a p-type approximately 1 OMEGA-cm crystal. Two reliable methods to overcome this difficulty are presented. One is to eliminate the multiple reflection effect physically by applying the p-polarized light incident on the sample surface at Brewster’s angle. The other is to eliminate the effect mathematically by a multiple reflection correction carefully taking account of the free-carrier absorption. Results obtained by the two reliable infrared absorption methods are consistent with a result by a high precision oxygen evaluation using secondary ion mass spectrometry.