Fourier Transform Infra-Red spectroscopy (FTIR) is commonly used to measure interstitial oxygen concentrations in silicon. The absorption coefficient αOX derived from the IR transmittance curve is converted to a concentration value through a multiplicative calibration factor γOX. Presently used calibration factors include 2.45 × 1017 cm−2 (ASTM F121−80), 3.03 × 1017cm−2 (JEIDA), and 4.81 × 1017 cm−2 (ASTM F121−76). These have been obtained from different analytical methods, including charged particle activation, helium carrier gas fusion-gas chromatography, vacuum fusion gas analysis, and photon activation combined with inert gas fusion. An alternative analytical method by secondary ion mass spectrometry (SIMS) is reported here. Oxygen ions of a known fluence are implanted into silicon samples of which oxygen content has been measured by FTIR. SIMS profiles of oxygen are performed using a cesium primary ion beam on a CAMECA IMS-3f. The result is a profile of the superposition of the implanted oxygen and the constant, bulk CZ oxygen. Integration of the implant signal followed by subtraction of the bulk signal can be used to calibrate the implant concentration from the implanted oxygen fluence, so that, in the same sample and during the same measurement, the SIMS calibration and FTIR calibration at the CZ bulk level can be directly compared. The γOX obtained from this method is 3.45 × 1017cm−2 and is closest to the JEIDA value of 3. 03 × 1017 cm−2.