Om is a mystical and sacred sound of Sanskrit origin. 

“All is OM: The whole universe is the syllable OM. Everything that was, is, or will be is, in truth OM. All else which transcends time, space, and causation is also OM. The Self or Atman, in the realm of sound is the syllable OM.”
-Yogi Ananda Sarasawathi

“OM is the Pravana mantra. It is also a revered greeting. It is a sacred recitation during prayers and japa. It is not merely a japa sound or recitation, a word or human language but something more than all human comprehension. OM comprehends all things. It is an auspicious beginning. The imperishable is OM, states Swami Krishnananda, and it is ‘all this’. Everything else, whatever be the past, present or future, is like an exposition, explanation or commentary on the meaning of this great Truth – the imperishable OM, indeed. This is how the Upanishad begins. OM ityetadaksharam idam sarvam: All this, whatever is visible, whatever is cognizable, whatever can come within the purview of sense perception, inference or verbal testimony, whatever can be comprehended under the single term, creation – all this is OM. As a cosmic vibration it is a chant not made, created or initiated by humans.”
-Yogi Ananda Sarasawathi