The word Om appears to have originated in the Mandukya Upanishad (one of a collection of ancient texts concerning philosophical concepts of Hinduism) which tells us that ‘Om is what has been, what is and what shall be’.
A – is the waking state
It contains all the words, mantras, and sounds of the world. Om is only physical form of God.
Om, therefore, represents both the unmanifest (nirguna) and manifest (saguna) aspects of God.
That is why it is called pranava—meaning that it pervades life and runs through our prana or breath.
Satender Kumar Mall