Disappearance of the divacancy (V2) and trivacancy (V3) complexes upon isochronal and isothermal annealing of electron irradiated Si:O crystals has been studied by means of deep level transient spectroscopy. The annealing studies have shown that the V2 and V3 defects are mobile in Si at T>200 °C and in oxygen-rich material are trapped by interstitial oxygen atoms so resulting in the appearance of V2O and V3O defects. The activation energies for diffusion of the V2 and V3 centers have been determined. Density functional modeling calculations have been carried out to investigate the migration and reorientation mechanisms of V3 in large silicon supercells. It is proposed that these comprise a sequence of transformations between V3(D3) and V3(C2v) configurations.