Om, AUM, Onkar is an ancient sound and is known as ShabdaBrahm or NaadBrahm. Entire creation that we see, feel and perceive around us and beyond is said to have originated from this sound. In the beginning of creation there was water all around but there was a sound prevailing that was the sound of OM, the primordial sound of the universe. It is a cosmic sound which prevails in this expanding span of universe due to vibrations of expansion.One may listen to this cosmic sound here.
This mysterious sound has been captured in Hindu meditation since the ancient Vedic traditions. It is a sound on which a Yogi meditates for Nirvana as well as the physical wellbeing of the creation.


OM syllable combined with a dot (Bindu) is meditated upon daily by the Yogis for physical wellbeing and the Nirvana. Therefore I bow before OM.OM is said to be the original primordial sound of universe, the AnahadNaad, the sound coming from void space, not produced by striking, from which the entire Universe has manifested. OM is the purest name of God, the sound of the Supreme Consciousness.OM occurs in the beginning of every prayer.e.g,any Invocation to gods must begin with this sound, e.g. we have OMNamahaShivaya, OM Shanti, OMbhurbhuvahswah etc.
It is also interesting that there is an OM mountain in Tibet Kailash Mansarovar region which has this `symbol inscribed in ice at the top of mountain. This has been in existence since the beginning of Himalayan mountain ranges. Just have a look:

om parvat_om

This sound has been considered most significant in ancient Hindu texts such as the Vedas and Upanishads as well as Shrimad BhagvadGeetaand is explained at so many places.
The word which all the Vedas proclaim,
That which is expressed in every Tapas(penance, austerity, meditation),
That for which they live the life of a Brahmacharin (abstinence),
Understand that word in its essence: Om! that is the word.
Yes, this syllable is Brahman,
This syllable is the highest.
He who knows that syllable,
Whatever he desires, is his. Katha Upanishad, 1.2.15-1.2.16 fromAtharva Veda
Shrimad BhagvadGeeta


Om syllable is ekaksharBrahmwhich is the only imperishable sound on which yogies should meditate.
Aum symbolism is very mysterious and is elaborated in the Mandukya Upanishad (Atharva Veda)as comprising of four parts- first part is sound ‘’A’’ for the physical world, the wakeful state, the second part is the sound “U” for the subtle world, the dream state, third part is the sound “M” for the blissful state, the heavenly region and the fourth is the Turiya state, the infinity, the Shunya(zero, silence) for the ultimate meditation stage of nothingness(Nirvikalpasamadhi). It is also pictorially explained below:


Even the Ganesha, the first divinity to be worshipped among the various divinities look like the syllable OM as shown here:
ganeshaum OM sound is also uttered before chanting any Sanskrit mantra. Some argue that a Sanskrit shloka becomes a Sanskrit mantra if OM precedes any shloka.

OM has been represented in various scripts as follows.
AUM Sound is the Sound Human Beings can articulate or hear. 'A' is Guttaral, from base of Stomach or Guts, 'U' is Labial and the 'M' is the Nasalized Vowel. Om sound energy created has to resonate from the bottom most part of the body's air cavity to the top most part of the air cavity.
AUM is also viewed as fourfold state of time: the past, the present, the future and the fourth state which transcends time.

It is said that the Gayatri Mantra which protects all the life forms of the world is the Veda Mata, the mother of Vedas or the essence of Vedas and AUM is the essence of that Gayatri Mantra. Thus reciting AUM is the ultimate prayer for a mankind, of which Hindus are a part.

In fact the use of Hindu text or Hindu prayer is only a popular term in general parlance, otherwise Hindu religion as such does not exist in our ancient scriptures where Sanatan (meaning eternal) Dharma or Sanatanreligion is the accepted terminology. The word Hindu has been imposed upon us by the Iranian Aryans who have a sound ‘h’for any ‘s’ sound in the beginning of a word. For Iranian Aryans the Aryans living on the other side of the Sindhu river were called Hindus as there were gross differences and disagreements in these two groups of Aryans. I have repeatedly used the term Hindu religion instead of the Sanatan religion just because it is more popular.

Hindu religion has been a growing and evolving religion (as against a static or dead religion) and it has moved along with the growth of the mankind. Our sages, manishies and thinkers have been assimilating newer concepts and ideas with the ancient thinking as the thinking processes of the mankind have been evolving. Without abandoning anything ancient, we have been assimilating and harmonising newly evolved thinking with the ancient scriptures and moved from Vedas to the Arnayaks, to the Samhitas, to the Upanishads, to the Smritis, and the Puranas, the Ramayana and Mahabharata the itihas and the prayers as a simplification process. The concept of Yoga, some argue, has gradually entered into Hindu mainstream thinking along with the unification of Shiva concept of Tibet and the Vishnu concept of South India. Lord Shiva is extolled as a great Yogi, Siddha and meditation on OM has followed along with that process.

For a Yogi, meditation on AUM has a component of Udgita pranayama where the Pranvayu, the upward directional breathing helps in raising the energy of consciousness through various energy chakras, transcending the vibrations of these energy centers one by one, from physical realm, to the subtle realm and to the blissful realm and ultimately the Shunya, the silence, the turiya stage which leads to Moksha. All the energy vibrations cease there and one attains a state of unification with the Brahm, the absolute spiritual Consciousness.
OM Shanti, Shanti, Shanti, may the meditation on AUM lead to Shanti, the eternal peace.