Defect structure of Czochralski grown (1 1 1) oriented silicon single crystals (Cz-Si), irradiated with fast neutrons (energy 5 MeV, dose 5×10 16 cm −2 ) and annealed at up to 1400 K, also under hydrostatic Ar pressure equal to 1.1 GPa, has been investigated by high-resolution X-ray diffraction and synchrotron topography at HASY Laboratory. The annealing, especially at 1270 and 1400 K, results in precipitation of interstitial oxygen and creation of extended defects. A thermally induced oxygen precipitation at high temperatures–pressures in neutron-irradiated Cz-Si reveals the irradiation-related history of investigated samples.