Om is the name of God/the universe/all of creation according to the ancient Indian Hindu texts known as the Upanishads. As the story goes, Brahman (absolute reality/God) was one and non-dual. Brahman thought, "I am only one - may I become many." This caused a vibration which eventually became sound. This sound was OM. Thus, OM is the closest vibration/sound that we can get to God.
In Sanskrit, OM translates to avati or Rakuati which means, "The one who protects or sustains." In essence: God.
So, OM is both a name for God and a sound that can be chanted to bring you closer to God. It is believed that we can "attune" ourselves to God - we can "experience" God by vibrating OM. This is something like an orchestral musician "attuning" their instrument to the rest of the orchestra.
OM is often spelled "AUM" and these three letters are usually assigned the following symbolism: A; creation or birth, U; blossoming or developing, M; completion or ending.
However, this is actually not all there is to OM. One way to understand OM is with the spelling AUMN. When saying or chanting AUMN, your mouth first opens and pours out the sound "A" (pronounced like the a in the word autumn). This is the beginning of the creation of the sound. Then, your mouth forms the "U" sound (pronounced like the u in the word up) and develops the sound further. After this, your mouth will close and complete the sound with the "M" (pronounced like the m in tomb). Lastly, your mouth forms the "N" sound (pronounced like the n in the word new) which brings the sound to the back of your mouth; returning the sound from where it first issued forth.
This is the same dynamics that are seen in the scientific theory of the creation, development, and return of the universe.
If you are new to the concept of chanting as a way to "attune" yourself to the universe (God), then I recommend starting with OM - chanted as in AUMN. Done daily, this simple chant will slowly bring you closer to a place of harmony - a place of balance - and, quite simply, will bring you closer to God.
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