Oxygen distribution in a Si crystal (100 mm in diameter) has been studied by the absorption method in the range of the absorption band of interstitial oxygen, λ = 5.81 μm. Large-scale fluctuations (∼1 cm) of the oxygen concentration (N 0) along the growth axis were determined. Depending on the melt height, the regions of the chaotic and quasiperiodic changes were established, as well as the region of the constant N 0 value, and their relation to turbulent, quasiperiodic, and stationary modes of melt convection in crystallization. The values of the critical Rayleigh number for the melt transition from stationary to quasiperiodic (3 × 103) and from quasiperiodic to turbulent (1.7 × 104) convection modes are determined for growth of silicon crystals by the Czochralski method. The dominating modes of N 0 concentration oscillations at two incommensurable frequencies, f 1 = 1.3 × 10−3 and f 2 = 6 × 10−4 Hz, are assumed to be related to the oscillatory transfer of oxygen from the walls of the quartz crucible to the crystallization front and restructurization of the convective flow pattern of the melt in the course of crystal growth.