ॐ OM also written as AUM is said to be the primeval sound through which the Lord created the entire universe. It is the root of all sounds in the universe. It pervades all creation. The five elements vibrate with these sounds. It is also called Pranava because it is the inner sound in all beings.
OM is not just a word . it is an intonation, like music . it transcends all barriers of space and time, age, race, and culture.
Omkar represents the total Godhead. it consists of three syllables A - U - M which are sounded progressively rising from the navel to the throat and ending on the lips. it subsumes all forms and names attributed to Divinity, all concepts of Time ( past, present, and future). AUM envisages the three attributes of the Supreme Being: creation, preservation, and Dissolution (Brahma Vishnu and Shiva ).
AUM symbolizes the four states of consciousness. the letter A represents the waking state, U represents the dream state, M represents deep sleep, and the whole word represents the fourth state (Turiya), which is the state of enlightenment, the awareness of one's own spiritual identity.
OM is the most important Mula (root) mantra and is thus chanted at the beginning of all sacred chantings. The name of God is great in itself, but even these are preceded by OM. When we start any auspicious work, we start with the word OM.
In Bhagavad Gita chapter 10, Verse 25, Lord Krishna said " of vibrations, I am the transcendental OM ."
When we chant OM, we create within ourselves a vibration that attunes with the cosmic vibration. The chanting has a profound effect on the body and mind of the one who chants and also on the surroundings.