"The Om sound is a high-pitched, electrical vibration that I'm sure you've heard - maybe on a quiet morning. There are ten levels - ten different manifestations of this sound. Chinese Taoists recognize two of these. One they call the hum of the dragon; the other, the growl of the tiger. The Om sound is the fundamental vibration of Life Itself. There is no work to be done with it, except, when you hear it, to be aware of it." (Swami Chetanananda in The Breath of God)
As the Swami says: no work to be done here, Om is a fundamental vibration of life itself, so if you are living (i.e. am-ing), you are om-ing.