OM, AUM
The most sacred single syllable in the entire literature of Hinduism is OM. This word is constantly on the lips of devout Hindus from the cradle to the grave. It stands for Brahman, both as personal and impersonal God.

Aum -Let us meditate on this syllable, which is the foundation of prayer.
The earth emerges from the waters; plants emerge from the earth; human beings emerge from plants; speech comes from human beings; and Aum comes from speech. Aum is the essence of all essences. It is the highest of the high. It is the ultimate.

Speech and breath combine to form Aum. Speech and breath are like a couple in coitus, and Aum is their offspring. Aum is the fulfillment of their desires.

The syllable Aum signifies assent, for we say it when we assent to something. And assent is nothing but fulfillment.
The syllable Aum is the call to knowledge. We sing the praises of this syllable, which is the key to every kind of knowledge.

Those who know the soul, and those who do not know the soul, stand side by side reciting this syllable. But knowledge and ignorance are quite different. Only when it is recited with knowledge and faith, and with awareness of the hidden connections, does it become truly potent.
Chandogya Upanishad I: I.

Omkara (pranava) is considered to be the sound incarnation of the Supreme Personality of God and is identical with the Supreme Lord. The Narada-pancaratra states: "When the transcendental sound vibration is practiced by a conditioned soul, the Supreme Lord is present on his tongue." The Atharva-veda and the Mandukya Upanishad both mention the importance of omkara. Omkara is said to be the beginning, middle, and end, and is eternal, beyond all material restrictions. Omkara is unlimited, transcendental, and indestructible.
OM represents the wholeness of the universe and the eternal vibratory sound of brahman (the universal soul) that permeates it. Indeed, Om is called shabda-brahman, the sound-form of brahman. All Hindu mantras generally begin and often also end with Om. It has been frequently mentioned in the Vedas and other scriptures of Hinduism.

Om is the mantra of assent. It means yes and affirms and energizes whatever we say after it. That is why all mantras begin with OM. OM is also the mantra of ascent and causes our energy to rise upward into the infinite. By reciting Om, a person gains access to the powers of the universe; and meditation on Om is said to lead to enlightenment and immortality. OM is expansive and increases the fire, air and ether elements, particularly ether. It also gives strength, protection and grace. It connects us with the guidance power of the inner Guru.
Aum is the supreme symbol of God. Aum is the whole. Aum affirms. Aum is the sound which expresses all truth.

The priest begins worship with Aum. Spiritual teachers begin their lessons with Aum; and their disciples open themselves to those lessons with Aum.
Those in whom Aum resides are unified with God.
Taittiriya Upanishad 1:8

The syllable Om, also known as Pranava, can also be spelled as AUM. According to one interpretation,
"A" stands for creation,
"U" stands for preservation,
and "M" indicates destruction or dissolution.

According to another interpretation, the three letters forming AUM indicate the three lokas (planes of existence) of this universe-both gross and subtle-Svarga (heaven), Martya (earth) and Patala (netherworld).
The following brief passage is one of the clearest of the countless references to OM which can be found anywhere in the Hindu scripture.

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.
Katha Upanishad I, ii, 15-17

Om reveals all and contains all and, like the Vedas, Om is revelation that comes through shruti (hearing). According to the Mandukya Upanishad, Om is both atman and brahman: it is the past, the present and the future, as well as all that transcends time.

The chanting of OM is generally practiced by impersonalists and those engaged in the mystic yoga process. By chanting OM and controlling the breathing perfectly, which is mostly a mechanical way of steadying the mind, one is eventually able to go into trance or samadhi. Through this system, one gradually changes the tendencies of the materially absorbed mind and makes it spiritualized. This process may take many years to perfect.

SYMBOLIC MEANING

Om - An Inquiry into its Aesthetics, Mysticism, and Philosophy
In the Chandogya Upanishad it is said:The essence of all beings is the earth.The essence of the earth is water.The essence of water is the plant.The essence of the plant is man.The essence of man is speech. The essence of speech is the Rigveda. The essence of Rigveda is the Samveda. The essence of Samveda is OM. Thus OM is the best of all essences, deserving the highest place. Visually, OM is represented by a stylized pictograph.A deeper insight into this mystic symbol reveals that it is composed of three syllables combined into one, not like a physical mixture but more like a chemical combination. Indeed in Sanskrit the vowel 'o' is constitutionally a diphthong compound of a + u; hence OM is representatively written as AUM.

Fittingly, the symbol of AUM consists of three curves (curves 1, 2, and 3), one semicircle (curve 4), and a dot.
The large lower curve 1 symbolizes the waking state (jagrat), in this state the consciousness is turned outwards through the gates of the senses.
The larger size signifies that this is the most common ('majority') state of the human consciousness.
The upper curve 2 denotes the state of deep sleep (sushupti) or the unconscious state. This is a state where the sleeper desires nothing nor beholds any dream.
The middle curve 3 (which lies between deep sleep and the waking state) signifies the dream state (swapna). In this state the consciousness of the individual is turned inwards, and the dreaming self beholds an enthralling view of the world behind the lids of the eyes.
These are the three states of an individual's consciousness, and since Indian mystic thought believes the entire manifested reality to spring from this consciousness, these three curves therefore represent the entire physical phenomenon.
The dot signifies the fourth state of consciousness, known in Sanskrit as turiya. In this state the consciousness looks neither outwards nor inwards, nor the two together. It signifies the coming to rest of all differentiated, relative existence This utterly quiet, peaceful and blissful state is the ultimate aim of all spiritual activity. This Absolute (non-relative) state illuminates the other three states.
Finally, the semi circle symbolizes maya and separates the dot from the other three curves. Thus it is the illusion of maya that prevents us from the realization of this highest state of bliss. The semi circle is open at the top, and does not touch the dot. This means that this highest state is not affected by maya. Maya only affects the manifested phenomenon. This effect is that of preventing the seeker from reaching his ultimate goal, the realization of the One, all-pervading, unmanifest, Absolute principle. In this manner, the form of OM represents both the unmanifest and the manifest, the noumenon and the phenomenon.

As a sacred sound also, the pronunciation of the three-syllabled AUM is open to a rich logical analysis.The first alphabet A is regarded as the primal sound, independent of cultural contexts. It is produced at the back of the open mouth, and is therefore said to include, and to be included in, every other sound produced by the human vocal organs. Indeed A is the first letter of the Sanskrit alphabet.The open mouth of A moves toward the closure of M. Between is U, formed of the openness of A but shaped by the closing lips.

Here it must be recalled that as interpreted in relation to the three curves, the three syllables making up AUM are susceptible to the same metaphorical decipherment.
The dream state (symbolized by U), lies between the waking state (A) and the state of deep sleep (M). Indeed a dream is but the compound of the consciousness of waking life shaped by the unconsciousness of sleep.
AUM thus also encompasses within itself the complete alphabet, since its utterance proceeds from the back of the mouth (A), travelling in between (U), and finally reaching the lips (M). Now all alphabets can be classified under various heads depending upon the area of the mouth from which they are uttered. The two ends between which the complete alphabet oscillates are the back of the mouth to the lips; both embraced in the simple act of uttering of AUM.

The last part of the sound AUM (the M) known as ma or makar, when pronounced makes the lips close. This is like locking the door to the outside world and instead reaching deep inside our own selves, in search for the Ultimate truth.But over and above the threefold nature of OM as a sacred sound is the invisible fourth dimension which cannot be distinguished by our sense organs restricted as they are to material observations. This fourth state is the unutterable, soundless silence that follows the uttering of OM. A quieting down of all the differentiated manifestations, i.e. a peaceful-blissful and non-dual state. Indeed this is the state symbolized by the dot in the traditional iconography of AUM.
The threefold symbolism of OM is comprehensible to the most 'ordinary' of us humans, realizable both on the intuitive and objective level. This is responsible for its widespread popularity and acceptance. That this symbolism extends over the entire spectrum of the manifested universe makes it a veritable fount of spirituality.
Some of these symbolic equivalents are:Colors: Red, White, and Black.
Seasons: Spring, Summer, and Winter.
Periods: Morning, Midday, and Evening.
States: Waking-consciousness (jagriti),
Dream (svapna), and deep-sleep (sushupti).
Spheres: Earthly, Heavenly, and Intermediary.
Poetic Meters: Gayatri (24 syllables),
Trishtubh (44 syllables), and
Jagati (48 syllables).
Veda: Rigveda (knowledge of the meters),
Yajurveda (knowledge of contents),
Samaveda (knowledge of extension).
Elemental Deity: Fire (Agni),
Sun (Aditya),
Wind (Vayu).
Manifestation of Speech: Voice (vak),
Mind (manas),
Breath (prana).
Priestly Function: Making offering, Performing ritual, and Singing.
Tendencies: Revolving, Cohesive, and Disintegrating.
Quality: Energy (rajas),
Purity (sattva), and
Ignorance (tamas).
Ritual fire: Of the home, of the Ancestors, and of Invocation.
Goddess: Amba, Ambika, and Ambalika.
Gods: Of the elements (Vasus), of the sky (Adityas), of the sphere-of-space (Rudras).
Deity: Brahma, Vishnu, Shiva.
Action: Creation, Preservation, and Destruction.
Power: of Action (kriya), of
Knowledge (jnana), and of
Will (iccha).
Man: Body, Soul, and Spirit. T
ime: Past, Present, and Future.
Stages of Existence: Birth, Life, and Death.
Phases of the Moon: Waxing, Full, and Waning. Godhead: Father, Mother, and Son.
Alchemy: Sulphur, Quicksilver, and Salt.
Buddhism: the Buddha, Dharma, and Sangha (three jewels of Buddhism).
Qabalism: Male, Female, and the Uniting intelligence.
Japanese Thought: Mirror, Sword, and Jewel.
Divine Attributes: Truth, Courage, and Compassion.

According to Indian spiritual sciences, God first created sound, and from these sound frequencies came the phenomenal world. Our total existence is constituted of these primal sounds, which give rise to mantras when organized by a desire to communicate, manifest, invoke or materialize. Matter itself is said to have proceeded from sound and OM is said to be the most sacred of all sounds. It is the syllable which preceded the universe and from which the gods were created. It is the "root" syllable (mula mantra), the cosmic vibration that holds together the atoms of the world and heavens. Indeed the Upanishads say that AUM is god in the form of sound. Thus OM is the first part of the most important mantras in both Buddhism and Hinduism, for e.g. Om Namoh Shivai and Om Mani Padme Hum.

In a further development of the mystic conception of AUM, the Mandukya Upanishad states:AUM is a bow,The arrow is the self,And Brahman (Absolute reality) is said to be the Mark.Another ancient text equates AUM with an arrow, laid upon the bow of the human body (the breath), which after penetrating the darkness of ignorance finds its mark, namely the lighted domain of True Knowledge. Just as a spider climbs up its thread and gains freedom, so the yogis climb towards liberation by the syllable OM.

Conclusion

The omnific and omniparous quality of OM makes it omnipresential, and in-omissible from any spiritual practice. As an omnipotent symbol, the yogi who penetrates its mystery is indeed truly omnicompetent and omnipercipient, and as an omniscient source, it is a virtual omnibus of sacred and mystical inspirations.