OM

OM
‘Om or Aum’ is a mystical or sacred Sanskrit syllable in the Hinduism, Jainism & Buddhism. Om is one of the most chanted sound symbols in India. It is said that it has a profound effect on the body and mind of the one who chants and also on the surroundings. Most mantras and vedic prayers start with Om.
All auspicious actions begin with Om. It is even used as a greeting - Om, Hari Om etc. It is repeated as a mantra or meditated upon. Its form is worshipped, contemplated upon or used as an auspicious sign.

Meaning
Om is the universal name of the Lord. It is made up of the letters A (phonetically as in "around"), U (phonetically as in "put") and M (phonetically as in "mum"). The sound emerging from the vocal chords starts from the base of the throat as "A". With the coming together of the lips, "U" is formed and when the lips are closed, all sounds end in "M".
The three letters symbolize the three states (waking, dream and deep sleep), the three deities (Brahma, Vishnu and Shiva), the three Vedas (Rig, Yajur and Sama) the three worlds (Bhur, Bhuvah, Suvah) etc. The Lord is all these and beyond.
The formless, attributeless Lord (Brahman) is represented by the silence between two Om Chants. Om is also called pranava that means, "that (symbol or sound) by which the Lord is praised". The entire essence of the Vedas is enshrined in the word Om. It is said that the Lord started creating the world after chanting Om and atha. Hence its sound is considered to create an auspicious beginning for any task that we undertake. The Om chant should have the resounding sound of a bell (aaooommm).
Four parts of OM: The OM Mantra has four parts. First, is the sound like "Ah," then "Uuuu," then "Mmmm," and finally the silence beyond the mantra. Thus, the mantra is also written as AUM, as well as OM. The three sounds, and the silence have the following meanings:
1. The A sound refers to the gross world.
2. The U part refers to the subtle realm.
3. The M refers to the causal plane, out of which the gross and subtle emerge.
4. The Silence after these three, refers to the absolute reality that is the substratum for each of the other three realities.
Start slowly in remembering the parts: When using the mantra in this way, it is easiest to begin by remembering the mantra very slowly, allowing time to be aware of each of the levels. Be aware of the four parts of the mantra separately (though forming a continuous sound), something like this:
AaaaaaUuuuuuMmmmmm.....Silence.....
1. When the "A" of OM mantra arises in the mind field, be aware of the gross world, including the objects of the world, the people, and your own physical being. Actually allow your attention to scan these aspects of the world. Do not get caught up in these objects; just be aware of this level of reality, and then let go of it, so as to go beyond.
2. When the "U" of OM mantra arises in the mind field, be aware, as best you can of the existence of the subtle or astral realm, including whatever objects you might intuit. As with the gross world, do not get caught up in these objects; just be aware of this level of reality, and then let go of it, so as to go beyond.
3. When the "M" of OM mantra arises in the mind field, be aware of the causal, the background stillness out of which the subtle and gross emerge, and into which they rest when not active. This can be difficult to imagine. It might help to think of it like being the canvas on which a picture is painted, or the screen on which a movie is projected.
4. When the Silence comes to the mind field, be aware of the absolute reality that permeates each of the other three layers. This fourth level of the mantra the absolute reality, is experienced in a shallow way at first, and only deepens with continued practice of contemplation and yoga meditation.
Four states of consciousness: The four parts of AUM also refer to the levels of waking, dreaming, and deep sleep, as well. The three sounds, and the silence have the following meanings:
1. The "A" sound refers to the waking state of consciousness.
2. The "U" part refers to the dreaming state of consciousness.
3. The "M" refers to the deep sleep state of consciousness.
4. The Silence after these three, refers to the witness consciousness that is observer of the other three states of consciousness.
The question can arise of why a spiritual seeker cares about the states of waking, dreaming, and deep sleep. It is said that the states of deep sleep, samadhi, and death are very close together, that they function at the same levels..
Begin by remembering slowly: As in the last practice, it is easiest to begin by remembering the mantra very slowly, allowing yourself time to be aware of each of the levels. With practice, it moves more quickly, as attention longs to rest in the silence.
1. When the "A" of OM mantra arises in the mind field, be aware of the waking state of consciousness. This does not mean thinking of the objects of the world, but rather, being mindful of your personal waking state, in relation to the world and your inner mental and emotional process. This simply means being aware of being awake.
2. When the "U" of OM mantra arises in the mind field, be aware of the dreaming state of consciousness. This does not necessarily mean that you are experiencing those dreams, but that you are mindful of the dream state that is beneath the waking state.
3. When the "M" of OM mantra arises in the mind field, be aware of the deep sleep state of consciousness. Be aware of how the mind is in complete stillness in that state, where there is no active thought process, no images, no pictures, and no words. All of these have come to rest in a dormant, formless form.
4. When the Silence comes to the mind field, be aware of the consciousness that permeates all of the other three states of consciousness. In other words, consciousness flows through the waking state, the dreaming state, and even the deep sleep state (although there is no active content in deep sleep). Imagine that you can somehow be witness to waking, dreaming, and deep sleep from a higher vantage point that is aware of all.
Focusing more inwardly: When using the OM mantra in this way, notice how very personal the practice is, in that you are consciously cultivating awareness of the levels of your own internal states. It has an in here focus. By contrast, the use of OM with the levels of gross, subtle, causal, and absolute reality (above) has more of a focus out there. Ultimately, they merge into one awareness.
Levels of consciousness: The four parts of AUM also refer to the levels of conscious, unconscious, and subconscious, as well as the consciousness that permeates these. (Note that some psychologies or systems use the words unconscious and subconscious in reverse, or use different terms. The meaning is what is important, not the particular choice of words). The three sounds, and the silence have the following meanings:
1. The "A" sound refers to the conscious level of mental functioning.
2. The "U" part refers to the unconscious level of mind.
3. The "M" refers to the subconscious level, the storage place of mind.
4. The Silence after these three, refers to the pure consciousness, which permeates the conscious, unconscious, and subconscious levels of awareness.
Begin slowly: Once again, it is easiest to begin by remembering the mantra very slowly. With practice, it moves more quickly, as attention longs to rest in the silence.
1. When the "A" of OM mantra arises in the mind field, be aware of the conscious level of mind. The easiest way to do this, is to contrast conscious to the unconscious. There is the conscious, that we are aware of here, in this external world, and there is the unconscious that is not seen in the deeper mind.
2. When the "U" of OM mantra arises in the mind field, be aware, as best you can, that your mind is presently functioning at an unconscious level as well. There is a tremendous amount of thinking process normally going on, that is out of view. This is what psychologists call primary process. It is going on behind the scenes, much like the microprocessor in a computer that is invisibly doing its work.
3. When the "M" of OM mantra arises in the mind field, be aware of the latent part of the mind, where all of the countless memories of people, objects, and day to day information is stored. It is there, like the information stored on a hard disk of a computer. Until it is needed, it just sits there in a latent, dormant form (It is actually in a formless form, somewhat like the data on the hard disk that is stored only as raw data, with no images as such). Be aware of that stillness, that is the storage of your deep impressions and habit patterns.
4. When the Silence comes to the mind field, be aware of the consciousness that permeates all of the other three levels. In other words, consciousness flows through the unconscious functioning. Consciousness is there in the latent part of the mind, even though those objects are not awakened into action at the moment. Eventually, this Silence expands to being the awareness of the other three states. It is at this stage that one is called a Seer, in that all of the other levels of mind are witnessed from this vantage point.