"The goal which all the Vedas declare, which all austerities aim at, and which men desire when they lead the life of continence is Om. This syllable Om is indeed Brahman. 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
"OM" is the most sacred syllable in Hinduism. "OM" is a sacred syllable representing Brahman, the impersonal Absolute of Hinduism — omnipotent, omnipresent, and the source of all manifest existence. Brahman, in itself, is incomprehensible; so a symbol becomes mandatory to help us realize the Unknowable. Om, therefore, represents both the unmanifest (nirguna) and manifest (saguna) aspects of God.That is why it is called pranava, to mean that it pervades life and runs through our prana or breath.
"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 (mulamantra), 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".
Vivekananda says," Ishvara (God) is the Atman as seen or grasped by the mind. His highest name is AUM --- Repeating AUM continually is the only true worship. It is not a word – It is god Himself. "(Vedanta Voice of Freedom p. 266).
AUM is an accepted symbol, adopted in practice by religious thoughts in India. The word AUM (AUM) is complete in it and signifies all functions. The Buddhists, the Sikhs and the Jains adopted it in their own ways.
According to the Mandukya Upanishad, "Om is the one eternal syllable of which all that exists is but the development. The past, the present, and the future are all included in this one sound, and all that exists beyond the three forms of time is also implied in it".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 Purānic Hinduism, Aum is the mystic name for the Hindu Trimurti, and represents the union of the three gods, viz. a for Brahma, u for Vishnu and m for Mahadev which is another name of Shiva. The three sounds also symbolize the three Vedas (Rigveda, Samaveda, Yajurveda).
According to Hindu philosophy , the letter A represents creation, when all existence issued forth from Brahma's golden nucleus; the letter U refers to Vishnu the God of the middle who preserves this world by balancing Brahma on a lotus above himself, and the letter M symbolizes the final part of the cycle of existence, when Vishnu falls asleep and Shiva has to breathe in so that all existing things have to disintegrate and are reduced to their essence to him. More broadly, Aum is said to be the primordial sound that was present at the creation of the universe. It is said to be the original sound that contains all other sounds, all words, all languages and all mantras.
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.
Om provides a dualistic viewpoint. On one hand, it projects the mind beyond the immediate to what is abstract and inexpressible. On the other hand, it makes the absolute more tangible and comprehensive. It encompasses all potentialities and possibilities; it is everything that was, is, or can yet be. It is omnipotent and likewise remains undefined.
During meditation, when we chant Om, we create within ourselves a vibration that attunes sympathy with the cosmic vibration and we start thinking universally. The momentary silence between each chant becomes palpable. Mind moves between the opposites of sound and silence until, at last, it ceases the sound. In the silence, the single thought—Om—is quenched; there is no thought. This is the state of trance, where the mind and the intellect are transcended as the individual self merges with the Infinite Self in the pious moment of realization. It is a moment when the petty worldly affairs are lost in the desire for the universal. Such is the immeasurable power of Om.