Sutra 1.27

When spelled out in Devanagari, the script used to write Sanskrit, Hindi, and several other languages, OM is written ओ३म्. This image shows OM as it is written in stylized Devanagari, Tamil, Kannada, Malayalam, Siddham, Chinese, Tibetan, and Balinese.

तस्य वाचकः प्रणवः॥२७॥ 
tasya vaachak-ha pranavuh-ha॥27॥

The expression of the stream of Pure Consciousness is OM (pranava). According to Vyas Houston it is the ‘primary sound frequency of creation heard as an inner ringing sound current.’ 

When the mind wanders in your day, bring it back to the sound 'OM' (pranava) and sound it distinctly as 'aa', 'o', 'mmm'. Open the sound with a big and wide 'aaa' in the belly and up through to the throat, where it rounds in the mouth, and echoes through the oral chamber as 'oh'. Continue the sound forward to the lips and to the head. Lightly closing the lips allow the sound to end with the completion of the exhalation sounding 'mmm' till you run out of breath. Listen with complete attention to the silence. Repeat.