Om is Pranava

Om is the Akshara, or the imperishable - i.e. God. Om is the
Universe, and this is the exposition of Om. The past, the present
and the future, all that was, all is, all that will be, is Om. Likewise
all else that may exist beyond the bounds of time, that too is Om.

Another word for Om is Pranava ('Nu' meaning praise and `Pra' meaning superior) Thus Om means the best prayer or praise.

The letters AUM also stand for the mantra `Tat Twam Asi' which means That which you are., Man's realisation of divinity within himself. This relieves man from the fragmentation of body, mind, psyche and spirit.

Om is composed of three sounds 'AH', `00', `MM'. It is the first word uttered and from it come all the alphabets and all the phonetics.

The letter `AH' begins the sound of OM and this sound is the first letter of the alphabet in all languages.

The letter `00' sustains the sound of OM. And finally `MM' ends the
sound of OM.

Hence Om stands for the Trinity - Brahmaa (The Creator), Vishnu (The Sustainer) & Mahesh (The Destroyer).

The Sanskrit `Om' and the Latin 'Omne' come from the same root.'Omkar' for the Sikhs and `Amen' for the Christians may also come from the same root.

The Mantra Om precedes all Hindu prayers. It is the original sound of Brahmaa. It may be practiced silently or vocally. The vibration of it should permeate the whole of our beings. It helps purify the physical, mental, emotional and spiritual aspects of our being.

"On the bow of the mystic syllable Om, place the arrow of the Self, point at the target of Brahman, with an undistracted mind stretch the bow and penetrate the mark," 

.Mundak Upanishad