Preamble

The Upanishad started with the question about this physical world. After  knowing the answer, the interest turned on from the physical plane to the  mental plane and thereafter to the spiritual plane. The fifth question asked by  the student, Satyakama, is about the meditation on AUM (some times written also  as OM). Before we go to the question raised by  Satyakama it is advisable to have a clear idea about what is meant by meditation  and also the significance of the syllable AUM as this section of the Upanishad  deals with pranava upasana or about meditation on AUM.

Meditation
Meditation means  the continuous flow of the mind towards Atman through the total exclusion of  all ideas foreign to It (The Atman). In meditation the mind becomes steady,  like the flame of a lamp set in a windless place. The principal disciplines for  meditation as laid down in Patanjali Yoga Sutras are yama and niyama which are methods of self-control at physical and mental levels. Yama includes  non-violence, truthfulness, non-stealing, chastity of body and mind, and  non-receiving of gifts. Niyama includes outer and inner purity, contentment,  austerity, study of scriptures, and devotion to God. The aspirant should  meditate on AUM with great love, regarding it as the symbol of Brahman. The  meditation should be practiced as a lifelong vow. For a more detailed  discussion on this, readers may refer to the author’s article entitled ‘Patanjali  Yoga Sutras’ in this website under the category ‘Yoga and Meditation’.

AUM
'Om' is the most sacred mystic  symbol. This single syllable incarnation is the most powerful and significant  of all the Mantras. The available literature upon the significance of this  Vedic Mantra is voluminous. Nowhere in the world can we meet with a more sacred  symbol that has got such a vast amount of significance. The word 'Om' has been taken as a symbol and as an aid to  meditation by spiritual aspirants from Vedic times which continue even today.  It is accepted both as one with Brahman and as the medium (the logos)  connecting man and God. This Holy Word is taught in the Upanishads. It  signifies Brahman, the divine substratum or ground of existence. It also  represents the Atman or the Self.

The sound of 'Om' is also called the  'Pranava', meaning that it is something that pervades life, or runs through  Prana or Breath. This cosmic sound is heard in deep meditation. Sound is  vibration, which, as the modern science tells us, is at the source of all  creations. God is beyond vibration, but vibration, being the subtlest form of  His creation, is the nearest we can get to Him in the physical world.  Speaking of 'Om',  Thaittiriya Upanishad says “Thou art the sheath of Brahman "- that is, Om is the container for the Supreme. So, invoking Om is invoking the Supreme. It is a sacred Mantra and is  to be chanted constantly. 

The very central theme of  Mandukya Upanishad is the syllable 'Om',  through which the mystery of Brahman is gathered to a point. The text of this  Upanishad first treats 'Om' in terms of the  Upanishadic doctrine of the 3 states of Waking, Dream and Sleep, but then passes  on to the fourth 'Turiya', thus transporting us into the classic  "Advaita-Vedanta ".

In 'Om'  there are 3 aspects: 
•The mere sound, the mere Mantra as pronounced by the mouth. 
•The meaning of the syllable, which is to be realized through feeling. 
•The application of 'Om' to our character, singing it in our acts and so through our life.

Om represents the Self, which is the Supreme non-dual  Reality. The Self is known in four states of consciousness - namely, the Waking  state, the Dream state, the Deep-Sleep state and the fourth state called the  'Turiya'. All these are represented in the three sounds of Om (A, U and M), and  the silence that follows and surrounds the syllable. This is illustrated in the  following table.

IMPORTANCE  OF TURIYA
The law of memory is that the person who remembers and  experiences must be one and the same individual, or else memory is impossible.  So, as we can remember all our experiences in all the three states, there must  necessarily be a single common factor, which was a witness of all the  happenings in all the three states. There must be some entity within us, who is  present in the waking world, who moves and illumines the dream, who is a  distant observer in the deep-sleep world, and yet who is not conditioned by any  of these 3 realms. This entity, conceived as the fourth state 'Turiya', is the  Real, the Changeless, and the Intelligent Principle.

If properly pronounced, this Om  will represent in itself the whole phenomenon of sound production. No other  word can do this. As Om is the nearest to God,  and is indeed the first manifestation of Divine Wisdom, this Om  is truly symbolic of God.

'Om' thus represents the entire manifested  world and the unmanifest, and also that which lies beyond both the manifest and  the unmanifest - the Brahman, which is the Changeless substratum for the  changing objects of the world of experiences.

'Om' is a mono syllable  prefixed to every Mantra. Without 'Om', no  sacred-chant has its power. A Mantra has no life in it without the addition of  the Pranava ‘Om’.

The Shloka "Om  iti ekaksharam Brahma--" of the Bhagavat Gita (8.13) explains the  importance of 'Om'. The meaning of the Shloka is:  “Anyone uttering the indestructible mono syllable 'Om',  the transcendental sound vibration of the Ultimate Truth, remembering me  continuously; thus relinquishing his body in this way achieves the Supreme  Goal".

Repetition of ‘Omkara‘is of different kinds. Generally it is uttered as part of some mantras like “Om Namasivaya” and similar other Vedic mantras as well as many hymns beginning with Omkara. So also sacrifices, acts of penance etc. begin with chanting ‘Om’.  But sometimes ‘Om’ is uttered and meditated upon independently.  This is called “Pranavopasana” and whether all people can adopt this upasana has been controversial. 

“Pranavopasana” means the repetition of Pranava (OM) by itself (not as part of, or in conjunction with mantras and rites) and the meditation upon its significance - the Supreme Brahman. According to traditional religious teachers this is reserved for Sanyasins only. The reason given is only those who have enough purity of mind and power of concentration can perform this Upasana in the proper way. The gist of the argument is that house holders and people like them lacking in mental purity and concentration on the Absolute are not qualified to perform Pranavopasana.