Early stages of thermal donor and oxygen cluster creation in Czochralski-grown silicon annealed at 450 degrees C were investigated. Enhanced (up to 1.2 GPa) argon pressure during anneals was applied to exert additional effect on the generation of oxygen-related clusters in silicon. Oxygen atoms participate in the formation at 450 degrees C of local oxygen agglomerates and thermal donors in the case of anneal under atmospheric pressure. High-pressure treatments enhance the rate of thermal donor generation and suppress oxygen accumulation in the agglomerates. Employment of the 450 degrees C anneal as a first step in the two-stages process (at 450-1033 degrees C) of oxygen cluster formation at high pressure leads to creation of clusters of oxygen precipitates and to more effective removal of interstitial oxygen atoms from the silicon matrix.