In the present work we have exploited different kinds of defect engineering (hydrogenation, irradiations at elevated temperatures etc) to enhance the formation of small oxygen clusters in Si crystals grown by the Czochralskitechnique. In these ways we manage to increase significantly the concentration of a defect giving rise to a local vibrational mode (LVM) band at 1006 cm-1. The highest concentration of the defect is achieved in samples first enriched with the vacancy-tri-oxygen complex (VO3) and then electron irradiated. It is found that an interaction of the radiation-induced Si self-interstitials with VO3 occurs, so resulting in the appearance of a complex incorporating three interstitial oxygen atoms. The results obtained give strong support for the assignment of the 1006 cm-1 band to the oxygen trimer. Three LVM bands positioned at 537, 723 and 1020 cm-1 are found to develop in a similar way to the 1006 cm-1 band and all of them are suggested to arise from the trimer.