OM is the name or symbol of God (or Brahman in Sanskrit).
OM stands for the essence of all sounds.
From OM this sense- universe has been projected.
OM is formed by adding the letters AUM.
A :represents the physical plane ( the gross manifestation)..
U : represents mental and astral planes, the world of spirits, all heavens (the subtle manifestation)..
M : deep sleep state; all that is unknown and beyond reach of the intellect (unmanifested state of Brahman).
OM therefore represents all.
The whole world has come from OM, rests in OM, and dissolves in OM.
A is the first letter of the alphabets, and M is the last syllabe in Sanskrit.
Thus A and M mean everything from A to Z. It is the Alpha and the Omega of everything, it represents everything from beginning to end.
As OM is the symbol of God or Brahman, it is the sacred monosyllable.
The vibration produced by chanting OM in the physical universe corresponds to the original vibration that first arose from the Hiranyagarbha (cosmic mind)
The Bible says: in the beginning was the word and the word was with God and the word was God. This is OM.
All languages and sounds come from OM.
Chanting of OM gives peace, calmness and tranquillity.
OM is the All-pervading sound that has come out of God. OM sound is the beginning of creation.
OM has emanated from the cosmic vibration.
OM is called Omkara by Sikhs, Jehovah by Jews, Allah Hoo by Muslims, Ahura Mazda by Zoroastrians.
The Japa of OM has tremendous influence on the mind.
Chanting of OM will bring remove tossing of the mind, shut down all worldly thoughts and generate sublime and soulstirring thoughts.
OM is the Lights of Light.

(From: meditation on OM by Swami Shivananda)