Effect of enhanced hydrostatic pressure (HP) on oxygen clustering in as-grown Czochralski silicon (Cz-Si) treated at up to 1000K – 1.6 GPa as well as on creation of defects in Cz-Si with SiOx precipitates, HP treated at 295K – 2 GPa and at 1580K – 1 GPa, has been investigated by infrared spectroscopy, electrical, photoluminescence and related structure – sensitive methods. Treatment of Cz-Si at 720–1000K resulted in enhanced generation of oxygen – containing nano-clusters exhibiting thermal donor activity while the HP treatment at 295K and 1580K – in creation of some additional defects (nonradiative recombination centres). Above effects are related to HP – induced creation of nucleation centres for oxygen clustering in initially “defect free” Cz-Si at 720–1000K and to generation of nano-defects at the SiOx /Si boundary in Cz-Si containing oxygen precipitates.