Power of "Om"

I read Vishnu Sahasranamam every day. Before every Nama a single syllable "Om" is added. Why is this so? Many a times I have questioned many people on this but not convinced with the answers. Ultimately, Gita gave me the justifying answer to this question as follows:

' Om Ithyekaksharam brahma vyaharanu mamanusmaram,
  Yah prayathi thyajanam deham, sayaathi, paramamgatham '

Gita says "Om" itself is God !

Purushasukham says :

'Om' is Parabrahma! 'Om' is Atman!

This clarified to me that the syllable 'Om' is the name of 'divine'.

I started repeating this "Om" whenever i am free. Also the chanting of "Om" creates vibrations which I believe will purify me because "God is pure".

I shall keep posting such short snippets on Gita frequently.