We chant Om often in yoga classes, but I don't think many students know the meaning or why we often open and close classes with chanting.
Om originates in the ancient vedic texts and definitions appear in both Patanjali's sutras and the Mandukya Upanishads. Om is the sound that represents all that is, pure consciousness and that which we can't know. It is the sound and representation of the ultimate reality, the divine.
The three sounds that make up "Om," A, U, M and the silence that follows represent four states of consciousness: waking, dreaming, consciousness and the higher self. Spoken together the sounds and silence unite these as one. We experience the vibrations of Om from abdomen, through the chest, to the head and beyond to our higher consciousness as a way to connect us to our higher self, our collective consciousness, our god.
I believe Om encompasses all belief systems as a prayer to god, a connection to consciousness or a centering of the self. The belief of the individual drives the experience of Om.
For more reading about the meaning and origin of the word, here are some useful web links: