In Sanatana (Hindu) dharma why "OM" is taken as a symbol of God?

It is said that " Word is God". If there is a symbol that is base to all symbols that is called the best symbol. During talking process we use our throat,tongue and palate. Is there any sound that resembles all sounds? Yes that is the Pranava or Omkaara.Om is combination of A,U,M. No part of tongue and palate are used for pronunciation of 'A'. This 'A' is beeja or base for Omkaara.Last one is 'M'.We pronounce this with our lips. From the starting point to ending point of mouth 'U' is pronounced.Like this 'Om' is indicating the process of pronouncing the words. So 'Om' is taken a a symbol of God.
" Swami Vivekananda "

The Bhagavadgeeta

" yatra yOgISwara kRshNO yatra paartha dhanurdaraha
tatra Sree rvijayOrbhooti dhruvaa neetirmatirmama "

Where lord Krishna and Arjuna are there there will be Victory and Richness.

Inner meaning:

Where Human effort and God's grace are combined there will be Victory and Richness.