We have used local vibrational mode (LVM) spectroscopy to monitor the formation of oxygen-related thermal double donors (TDDs) at 450 °C and their annihilation at 650 °C in carbon-lean Czochralski-grown (Cz-) Si crystals. A few samples were treated at 650 °C under high hydrostatic pressure. It is found that the annihilation of TDDs at 650 °C results not only in a partial recovery of the interstitial oxygen, but also in the appearance of a number of new O-related LVM bands in the range 990–1110 cm−1. The positions of these lines and their shapes are identical to those observed for Cz-Si irradiated with electrons or neutrons and annealed at 600–700 °C. Since the lines appear upon annealing out of V O3 and V O4 defects in irradiated samples, they are suggested to arise from V Om (m>4) complexes. In both kinds of samples, pre-annealed and pre-irradiated, the new LVM bands disappear upon prolonged annealing at 650 °C while enhanced oxygen precipitation occurs. The V Om defects are suggested to serve as nuclei for oxygen precipitates developing at around 650 °C. High hydrostatic pressure is found to enhance further (up to 4–5 times) the oxygen precipitation process at 650 °C in the samples pre-annealed at 450 °C.