The evolution of the defect structure and electric properties of the oxygen-implanted silicon samples subjected by thermal treatment in the temperature range from 700 to 1100 °C is investigated using transmission electron microscopy (TEM) and space charge spectroscopy. It is established that embedded positive charge of oxygen precipitates (OPs) decreases inversely proportional to their size. This fact gives evidence about localization of the charge in thin nonstoichiometric shell of OP. Annealing at 700 °C results in the formation of nanosized defects and in an inhomogeneous electric structure of the implanted region: the embedded positive charge is localized in the near-surface vacancy-rich part of the implanted region, whereas the strongly compensated layer is formed in self-interstitial-rich deepest part of the implanted region.