from wildmind.org Like many mantras, this one begins with "Om". Om has no meaning, and its origins are lost in the mists of time. Om is considered to be the primeval sound, the sound of the universe, the sound from which all other sounds are formed.
In the Brahminical tradition, from where Buddhism undoubtedly obtained mantra practice, Om is not just the universal sound, but the sound of the universe itself. For example in the (non-Buddhist) Mandukya Upanishad, it is said: Om! This syllable is this whole world. Its further explanation is: The past, the present, the future everything is just the word Om. And whatever else that transcends threefold time that, too, is just the word Om. Om is therefore a sound symbolizing reality. It represents everything in the universe, past, present, and future. It even represents everything that is outside of those three times. It therefore represents both the mundane world of time in which the mind normally functions, and the world as perceived by the mind that is awakened and that experiences the world timelessly. It represents both enlightenment and non-enlightenment. You could regard Om as being the equivalent of white light, in which all of the colors of the rainbow can be found. One Sanskrit-English dictionary says the following: "A word of solemn affirmation and respectful assent , sometimes translated by `yes, verily, so be it' (and in this sense compared with Amen); it is placed at the commencement of most Hindu works, and as a sacred exclamation may be uttered at the beginning and end of a reading of the Vedas or previously to any prayer; it is also regarded as a particle of auspicious salutation [Hail!]; Om appears first in the Upanishads as a mystic monosyllable, and is there set forth as the object of profound religious meditation, the highest spiritual efficacy being attributed not only to the whole word but also to the three sounds A, U, M, of which it consists."
