In addition to the well-known spectrum of Czochralski-grown silicon, two infrared bands at 560 and 648 cm−1 are found to arise from the interstitial oxygen in silicon. The assignment of the bands is performed using the studies of their oxygen isotope shift and temperature dependence. The 560 cm−1 band is attributed to the hot counterpart of the mode at 518 cm−1 and the 648 cm−1 band to the combination of the far-infrared mode at 29 cm−1 with the Raman active mode at around 600 cm−1.