We investigated, experimentally as well as theoretically, defect structures in electron irradiated Czochralski-grown silicon (Cz-Si) containing carbon. Infrared spectroscopy (IR) studies observed a band at 1020 cm−1 arisen in the spectra around 300 °C. Its growth occurs concomitantly with the decay out of the well-known vacancy-oxygen (VO) defect, with a Local Vibrational Mode (LVM) at 830 cm−1 and carbon interstitial-oxygen interstitial (CiOi) defect with a LVM at 862 cm−1, in silicon (Si). The main purpose of this work is to establish the origin of the 1020 cm−1 band. One potential candidate is the carbon interstitial-dioxygen (CiO2i) defect since it is expected to form upon annealing out of the CiOi pair. To this end, systematic density functional theory (DFT) calculations were used to predict the lowest energy structure of the (CiO2i) defect in Si. Thereafter, we employed the dipole–dipole interaction method to calculate the vibrational frequencies of the structure. We found that CiO2i defect has an LVM at ~1006 cm−1, a value very close to our experimental one. The analysis and study of the results lead us to tentatively correlate the 1020 cm−1 band with the CiO2i defect.