Om, the mystic syllable of A-u-m is venerated by the Barahmans as well as by the Buddhist.The devotees regard it to be too sacred to be uttered aloud only formed by the lips. It is written that when all was void the triliteral syyllable Aum became mainfest.

Most religions indicate that creation began with sound-- In the beginning was the word...For the Hindus & Buddhists, Om is the primordial sound, the first breath of creation, the vibration that ensures existence. Om sign signifies God, Creation, & the One-ness of all creation.

In Buddhism

Adi Buddha at his will proceeded from OM. In that Aum, Adi Buddha who is present in all things formless and passionless, possesses the TriRatna. It is also written that Adi Became mainfest in the greatest Sunyata (void) as the letter A. It was also written that when all was sunita, Prajna Devi (Adi Dharma) was revealed out of Akasa with the letter 'U'. The Vija mantra of Sangha is "M'. Thus the letter A-u-m are that vija mantra of Buddha. Dharma and Sangha V. Tri-Ratna

The mantra generally begis with Om and ends with hum. The most widely known mantra is the six syllabe one of Avalokiteswara
' Om Mani Padme Hum'.

In Hinduism

The Om symbol is a sacred syllable representing Brahman, the impersonal Absolute — 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.

Aum is said to be the essence of all mantras, the highest of all matras or divine word (shabda), brahman (ultimate reality) itself. Aum is said to be the essence of the Vedas.

By sound and form, AUM symbolizes the infinite Brahman (ultimate reality) and the entire universe.

A stands for Creation

U stands for Preservation

M stands for Destruction or dissolution

This is representative of the Trinity of God in Hindu dharma (Brahma, Vishnu and Shiva)

The three portions of AUM relate to the states of waking, dream and deep sleep and the three gunas (rajas, satva, tamas)

The three letters also indicates three planes of existence, heaven (swarga), earth (martya) and netherworld (patala)

All the words produced by the human vocal organ can be represented by AUM. A is produced by the throat, U & M by the lips

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.