Omkara is the primordial sound from which the whole universe was created. Om is also called the shabda Brahman – God as sound/vibration. Yoga theory says that by following the thread of Om during meditation back to more and more subtle levels of awareness, the yogi regains union with Brahman.
There are many references to Om, or Omkara, in the Upanishads and the Yoga Sutras Patanjali. In the Upanishads, the purpose of chanting Omkara is stated as follows:
"What world does he who meditates on Aum until the end of his life, win by That? If he meditates on the Supreme Being with the syllable Aum, he becomes one with the Light, he is led to the world of Brahman [the Absolute Being] Who is higher that the highest life, That Which is tranquil, un-aging, immortal, fearless and supreme."
-Prashna Upanishad 5:1,5,7