OM
is the sacred monosyllable - the oldest of the most venerated of all mantras. It is also called the root, or mula-mantra. Often most vedic mantras and prayers begin with OM.

The OM sound symbolizes universal divinity and is referred to as the sound of the soundless absolute or the sound of creation. It is through this sound that one focuses the attention to the point from where the finite consciousness can be transcended.

The Mandukya-Upanishad which is dedicated to the analysis of OM, opens with the passage:

OM! This syllable is the whole world.

The past, the present, and the future - everything is but the sound OM.
And whatever else that transcends triple time - that too is the sound OM.

It explains that OM is composed of three parts, namely A (as in "about"), U (as in "put") and M (as in "mum") which signifies the three states of consciousness: waking, dream and deep sleep; the three Vedas: Rig, Yajur and Sama; the three worlds: past, present and future. The formless, attribute less Absolute (Brahman) is represented by the silence between the repeated OM chants.

The chanting of OM should have a resonating sound of a bell... aaooommm. It fills one with peace and promotes focus.