"OM" (Hindi: ॐ or ओं or ओ३म् ) is the most sacred syllable in Hinduism."OM" is a Sanskrit word, which is said to be the original primordial creative sound from which the universe and all of creation first manifested. It is the root syllable (mula mantra), the cosmic vibration that hold together the atoms of the world and heavens. While pronounced OM, certain Sanskrit scholars state that it is written as AUM.
The vibration of OM contains all the concepts of Him and all His manifestations. No other mantra or symbol gives greater calmness and divine attunement of the mind to God.
Om is comprised of three sounds:
The sound starts from the base of the throat as "Ah" with the coming together of the lips, "Ooooh" is formed and when the lips are closed, the sound ends in "Mmmmm" - Try to extend the mmm part of the OM, despite the natural tendency to shorten it.
Swami Vivekananda Says, "To know the Om is to know the secret of the universe."
Swami Sivananda Radha says, The cosmic sound AUM, or its condensed form, OM, is the origin of all other sound. OM is everything. It is the name of God.
The Mandukya Upnishad, composed between 400 BC and 200 BC, is dedicated to analysis of the theology and meaning of OM. The root syllable is described as "bow" which fires the"arrow" of the self (soul/atma) at the "target" of the absolute(brahman).
In Maitrayaniya Upnishad, Om is the "Sound of the soundless absolute."
According to Katha Upanishad -- I, ii, 15-17 : The goal which all the Vedas declare, which all austerities aim at, and which men desire when they lead the life of continence, I will tell you briefly: it is OM. This syllable OM is indeed Brahman. This syllable is the Highest. Whosoever knows this syllable obtains all that he desires. This is the best support; this is the highest support. Whosoever knows this support is adored in the world of Brahma.
In The Gospel of Sri Ramakrishna -- The sound OM is Brahman. The rishis and sages practiced austerity to realize that Sound-Brahman. After attaining perfection one hears the sound of this eternal Word rising spontaneously from the navel. "'What will you gain', some sages ask, 'by merely hearing this sound?' You hear the roar of the ocean from a distance. By following the roar you can reach the ocean. As long as there is the roar, there must also be the ocean. By following the trail of OM you attain Brahman, of which the Word is the symbol. That Brahman has been described by the Vedas as the ultimate goal."
The pictogarph of "OM" is often said to be a manifestation of brahman, the self that trascends and unites the "trimurti" or the "Trinity of God in Hindu dharma (Brahma, Vishnu and Shiva)". It is also a representation of time "Past, Present and Future".
The Mandukya Upanishad associates it with the four states of conciousness--waking, sleeping, dreaming and turiya (which is transcendental self beyond the concious mind).
"Om" as a Sound :
Of all the sacred sounds on this earth, the one that is most often chanted is the OM. Undoubtedly, at any given moment, there is someone, somewhere chanting this sacred mantra. Sound of "OM" symbolizes the supreme source. To many, the sound of OM is synonymous with the energy of peace.
In the Vedas, AUM is the sound of the Sun, the sound of Light. It is the sound of assent (affirmation) and ascent (it has an upwards movement and uplifts the soul, as the sound of the divine eagle or falcon.
According to Bhagavad Gita By vibrating the sacred syllable Om, the supreme combination of letters, if one thinks of the Ultimate Personality of Godhead and quits his body, he will certainly reach the highest state of "stateless" eternity. In Yogic practice, "OM" is a fundamental part of the techniques of auditory meditation.