Oxygen-vacancy complexes formed after rapid thermal annealing in silicon wafers were investigated by FTIR spectroscopy at 6 K. It was found that VO4 is the only detectable complex. The concentration of VO4 complexes increases with increasing temperature of RTA treatment in the temperature range between 1250°C and 1400°C. The concentration at maximal temperature is equal to 4.5 × 1013 cm−3. The experimental results were compared with concentrations of VOn complexes in silicon wafers obtained using ab-initio calculations combined with rate equation modelling. The simulated concentration of VO4 corresponds well to the measured concentration. The bulk microdefect density increases with increasing VO4 concentration. The vacancies stored in VOn complexes after RTA are slowly released during further annealing and enhance oxide precipitation.