Om is unequivocally regarded as the oldest mantra known to the sages of India. However it’s origins are obscure, not helped by the fact that Vedic and Yogic traditions were based on verbal transmission of secret knowledge and only written down relatively recently, and it was even more so with such a powerful and sacred sound such as ‘OM’. In his essay ‘The Sacred Syllable OM’, Georg Feuerstein describes various speculations about the origins of OM which I shall briefly summarise here:
Max Müller – (editor and translator of the Rig-Veda) – OM might be a contraction of the word avam, which is a prehistoric pronominal stem, pointing to distant objects.
Swami Sankarananda – OM derives from the Vedic word soma. Through the influence of the Persians, who did not pronounce the letter s, the word soma was changed to homa and subsequently was shortened to om. This relates to accepted relationship between soma and om. There is more information about Soma in Appendix 2.
Vihari-Lala Mitra – Equated the Greek word on (“being”) with Om. This can be accepted philosophically as Om is the symbol of That Which Is, or Brahman, but is a weak argument linguistically.
Traditionally – Most spiritual authorities regard Om as the vocalisation of an actual sound, or vibration, which pervades the universe and is audible to those in higher states of consciousness. It is also known as the shabda-brahman or sonic absolute.
Patanjali explains that Om is the symbol of the Absolute: tasya vãcakah praņavah – ‘His symbol is the praņava (om)’ and that taj-japas-tad-artha-bhavanam – ‘the recitation of that [syllable] [leads to] contemplation of it’s meaning’. And the reason why we would want to recite Om: Tatah pratyakcetana-adhigamo’py-antaraya-abhavas-ca – ‘thence follows the attainment of habitual inwardmindedness and also the disappearance of the obstacles.’
OM was traditionally only used in the context of ritual worship, a secret sound, it was only communicated by word of mouth from teacher to student. The early Upanishads often only referred to it indirectly as udgîtha (up sound) and the pranava (pronouncing). However the Brihad-Âranyaka, Chândogya, and Taittirîya Upanishads occasionally mention Om by name as om (or aum) and om-kâra (“om making,” meaning the “letter om”).
The Chândogya Upanishad is the first to spell out the equation between udgîtha and pranava. Georg Feuerstein comments in his essay, ‘The sacred syllable OM’, ‘Perhaps these two terms came in vogue because for unknown reasons om had, by that time, spread beyond the sacred domain and begun to be used in the sense of “Yes, I agree.” The first record of this usage is in the Brihad-Âranyaka-Upanishad (3.9.1) itself, where Om is employed seven times in this manner. Indeed, the Chândogya-Upanishad (1.1.8) clearly states: “That syllable is a syllable of assent, for whenever we assent to anything we say aum [= om].”.
In the Mândûkya-Upanishad the three constituent parts (mâtrâ) of the syllable Om are explained: Namely a + u + m, symbolizing past, present, and future, as well as waking, dreaming, and deep sleep. He also spoke of a fourth part that transcends the other three and concludes with the statement that Om is the Self (âtman), saying, ’He who knows this enters the Self with the self-indeed, he who knows this!’
This scripture begins with the question: What should one meditate on? The answer is: the syllable Om, which symbolises the supreme Absolute (brahman). The text describes four constituent parts of Om, each having its own symbolic correlations as follows
1. the sound a — earth – ric (hymn of praise) — Rig-Veda — Brahman — Vasus (a class of eight deities) — gâyatrî meter — gârhapatya fire — red — dedicated to Brahman;
2. the sound u — atmosphere — yajus (sacrificial formula) — Yajur-Veda — Vishnu — Rudras (deities governing the region between earth and heaven) — trishtubh meter — dakshina fire — bright — dedicated to Rudra;
3. the sound m — heaven — sâman (sacred chants) — Sâma-Veda — Vishnu — Âdityas (deities connected with the Goddess Aditi, symbolizing primordial infinity) — jagatî meter — âhavanîya fire — black — dedicated to Vishnu;
4. “half-part” (ardha-mâtra) — Atharvan songs — Atharva-Veda — fire of universal destruction — Maruts (deities of the mid-region who are especially associated with the wind) — Virât — lightning-like and multicolored — dedicated to Purusha.
This Upanishad further states that the om sound is called om-kâra because it sends the currents of the life force upward (ûrdhvam utkrâmayati) and that it is called pranava because it makes all the life currents bow down (pranâmayati) before it. The text concludes by stating that the Om sound is Shiva.
Om is the original seed syllable, the source of all others. The Mantra-Yoga-Samhitâ (71) calls it the ‘best of all mantras,’ adding that all other mantras receive their power from it. Thus Om is prefixed or suffixed to other mantras:
* Om namah shivâya. (Om. Obeisance to Shiva)
* Om namo ganeshâya. (Om. Obeisance to [the elephant-headed] Ganesha.)
* Om bhûr bhuvah svah tat savitur varenyam bhargo devasya dhîmahi dhiyo yo nah pracodayât. (Om. Earth. Mid-region. Heaven. Let us contemplate the most excellent splendour of Savitri, so that He may inspire our visions.) – This is the famous Vedic gâyatrî-mantra.
* Om sac-cid-ekam brahma (Om. The singular Being-Consciousness, the Absolute) – The Mahânirvâna-Tantra (3.13) calls this the most excellent of all mantras, which is said to bestow not only liberation but also virtue, wealth, and pleasure. Unusually It is said to be suitable for all practitioners and does not require the usual rituals and practices (including numerous repetition) before it is given. The Mahânirvâna-Tantra (3.24,26) claims ‘Merely by receiving the mantra, the person is filled with the Absolute…guarded by the brahma-mantra and surrounded with the splendour of the Absolute, he becomes radiant like another sun for all the planets etc.’
The rishis, or ancient seers, taught that all of creation is a manifestation of the primordial sound Om. Reflected in an interpretation of the word universe—’one song’—Om is the seed sound of all other sounds.
 While ayam pointed to nearer objects. Avam may have become the affirmative particle om, just as the French oui arose from hoc illud.” This obscure comment refers to the fact that om, in addition to its sacred significance, came to be used in the prosaic sense of ‘Yes, I agree’
 Chândogya-Upanishad (2.23.3), is ‘all this (idam sarvam).’ OM is the universe as a totality, not a conglomerate of individual parts, as we experience it in our ordinary state of consciousness. Thus Om is the primordial sound that reveals itself to the inner ear of that the adept.