Interactions between the oxygen dimer (O2i) and silicon self-interstitials (I) and vacancies (V) have been studied in Czochralski-grown silicon (Cz–Si) crystals using infrared absorption and deep level transient spectroscopies. The focus in this report is on reactions of O2i with I. The first step in this interaction is found to be the formation of a self-interstitial-dioxygen centre (IO2i) with oxygen-related local vibrational mode (LVM) bands at 922 and 1037cm−1. During the second formation step, another centre, I2O2i, with LVM bands at 918 and 1034cm−1 is suggested to appear. A Si-related band at about 545cm−1 is also assigned to both the IO2i and I2O2i centres. The IO2i centre is found to be electrically active with an acceptor level at Ec−0.11eV. The both defects, IO2i and I2O2i, are stable at room temperature and anneal out at about 400 and 550K, respectively.