As it is greatest of all mantras
Symbolism. Written by: Jacob N. Kinnard. One of the most common symbols in Hinduism is the Sanskrit letter om (or aum). This is the understood not only as the first letter (or sound) of the sacred alphabet, but the first sound in the cosmos that led to creation. It is thus the first principle of the universe. In Hinduism, Om (also spelled Aum) is a Hindu sacred sound that is considered the greatest of all mantras. The syllable Om is composed of the three sounds a-u-m (in Sanskrit, the vowels a and u combine to become o) and the symbol's threefold nature is central to its meaning. It represent several important triads: The Om (or Aum) sign is the main symbol of Hinduism. Most religions indicate that creation began with sound-- In the beginning was the word... For the Hindus & Buddhists, Om is the primordial sound, the first breath of creation, the vibration that ensures existence. Om or Aum is of paramount importance in Hinduism. This symbol (as seen in the image on the right) is a sacred syllable representing Brahman, the impersonal Absolute of Hinduism — omnipotent, omnipresent, and the source of all manifest existence. Aum or Om (in Devanagari ॐ) is one of the most sacred symbols in Hinduism. In Sanskrit known as praṇava (प्रणव) lit. to sound out loudly or oṃkāra (ओंकार) lit.  oṃ syllable). Hindus consider Aum to be the universal name of the Lord and that it encompasses all of creation.
http://www.patheos.com/Library/Hinduism/Ritual-Worship-Devotion-Symbolism/Symbolism http://www.religionfacts.com/om http://www.lotussculpture.com/my_articles_om.html http://hinduism.about.com/od/omaum/a/meaningofom.htm https://en.wikibooks.org/wiki/Hinduism/Religious_Symbols_of_Hinduism
why is aum the symbol for hinduism