nuryoga:

Om is not just a sound or vibration. It is not just a symbol. It is the entire cosmos, whatever we can see, touch, hear and feel. Moreover, it is all that is within our perception and all that is beyond our perception. It is the core of our very existence. If you think of Om only as a sound, a technique or a symbol of the Divine, you will miss it altogether. ….. Om is the mysterious cosmic energy that is the substratum of all the things and all the beings of the entire universe. It is an eternal song of the Divine. It is continuously resounding in silence on the background of everything that exists.

The book Om Chanting and Meditation - Amit Ray