The intensities of infrared absorption due to the asymmetric stretching vibrations of interstitial oxygen atoms in Ge crystals enriched with and isotopes have been compared with oxygen concentrations determined by means of secondary ion mass spectrometry. For Ge samples with oxygen content less than 5×1017 cm−3, a good correlation has been found between the values of oxygen concentration and the values of the absorption coefficient at the maximum of the related absorption band at 855.6 cm−1 with a proportionality coefficient CO = 1.05×1017 cm−2.