The symbol for 'OM' does not occur in the Rig Veda (circa 1500 BCE). OM isn't mentioned in the Rig because there was no written text back then (archaic Sanskrit) - the symbol for OM was added much later.
It is found mentioned in the Chandogya Upanishad, (udgitha), which was obviously composed after the historical Buddha's passing. OM is alluded to in Patanjali's Yoga Sutra (pranava) (circa 200 BCE), and in the Bhagavad Gita (circa 100 CE). The Gita appeared later than the great movement represented by the early Upanishads. So, if OM had been known before 400 BCE, it would have been mentioned by the Buddha, Shakya the Muni. The symbol OM isn't really a bija mantra at all; bijas didn't come into use until the age of the tantrics (circa 100 CE). If you inspect the archaic Sankrit of the Rig Veda, you will not find the OM symbol. OM was added to the text during the time of their arrangement by Pannini. For example, you won't find the bija mantra 'phat' in the Rig Veda either. So, how did we get OM? It's just the symbol of the Transcendent, an expression that was added after the age of tantrism, after the invention of writing. OM is the mantra of assent. It means yes and affirms and energizes whatever we say after it. Obviously, there were no esoteric symbols in Sanskrit before writing! The first instance of writing in India occurs on the Sarnath Pillar erected by Asoka (circa 200 BCE). There is no OM symbol in the Saraswati civilzation (circa 2400 BCE). OM is a verbal expressions, not part of the original Gayatri Mantra. So, OM is not a primordial symbol - it is an invention of tantrics that was added later. OM is merely a salutation, just like 'Amen" is used in Hebrew. For example, there's no Amen expression in the Old Testament - it is added when the passage is recited, as an affirmation. According to Mircea Eliade, quoting the Tantras - tantric sadhana explains the importance of sexuality. Tantrics teach control of the temporal rhythms by disciplining respiration, thus escaping the domination of time: "The flesh, the living cosmos, and time are the three fundamental elements of tantric sadhana." Work cited: 'Yoga: Immortality and Freedom' by Mircea Eliade Princeton, Bollengen Foundation Second Edition 1969 p. 264 The standard text on Yoga; scholarly; definitive, by the author of 'Shamanism', 'The Myth of the Eternal Return', 'History of Religious Ideas', etc. Other titles of interest: 'Myths and Symbols in Indian Art and Civilization' Heinrich Robert Zimmer, Joseph Campbell Princeton University Press, 1972
