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.

Karma Yoga

Karma yoga is one ofthe four yogas that lordSri Krishna told in Bhagavadgeeta. In two slokas of Gita we can understand the secret of Karmayoga.Those are

1. You have right to do the work but not to have interest on the fruits of that work.It doesnt mean that not to do the works.

2. You should do the work with unattachment and with interest.It means at anytime during the work you should able to comeout of that work.

We can say the worker ,working in her owner's house as an example for second sloka.She serves the children of owner as if they are her child but all her interest is on her own house.It doesnt mean that she cannot do her owner's work interestly.She will do her owner's work interestly too.So we may be busy in doing the worldly works but we shouldnot forget the Almighty.This is secret of Karmayoga.If one practices it heartfully he can easily getrid of Samsara.