http://www.atmajyoti.org/
Swami Nirmalananda Giri
Here some excerpts:

Om Yoga
This sacred syllable is spelled out either as Om or Aum, but It is usually written in the ideogrammatic forms: or

It is most important in repeating Om to pronounce the O correctly. It should be pronounced with a long O.
Om is also considered to be formed of the three letters a, u, and m
In Sanskrit, when a and u are combined they produce the sound of o. However, this only applies to verbal speech. In mental “speaking” we make the pure sound of o, not a and u together. So the inner Om is only two letters, not three.
Om should be intoned giving full value to both the O and the M.
1) Sit upright, comfortable and relaxed.
2) Breathe naturally.
3) Gently and without any strain turn your eyes upward as though looking at a point far distant. Then gently close them - do not squeeze them tight.
4) Be aware of your breath naturally (automatically) flowing in and out as you breathe through your nose.
5) Now begin mentally (do not make a sound!) intoning (“singing” on a single note) Om once throughout each inhalation and once throughout each exhalation. Fit the intonations to the breath-not the breath to the intonations. If the breath is short, then the intonation should be short. If the breath is long, then the intonation should be long. Make sure the O and the M get approximately “equal time”.
6) For the rest of your meditation time keep on intoning Om in this manner - in time with the breath - listening to your inner intonations of Om. This enables you to enter effortlessly into the Witness Consciousness that is your finite spirit within the Infinite Spirit that is God.
“Amazingly Om can become a silent sound.”
7) In time your inner mental intonations of Om may change to a more mellow or softer form, even to an inner whispering, but Om is always fully present and effective. Your intonations may even become silent, like a soundless “mouthing” of Om, yet you will still be intoning Om in your intention. Amazingly Om can become a silent sound, as you can experience for yourself. But of this be sure: Om never ceases. Never.
8) In the same way you will find that your breath will also become more subtle and refined, and slow down. Sometimes your breath can become so light that it almost seems as though you are not breathing at all. At such times you may perceive that your inhaling and exhaling are more like a magnetic pull in and out instead of actual breath movements. This occurs as the prana that produces the breath switches back and forth in polarity from positive to negative.
9) In Om Yoga we do not deliberately concentrate on any particular point of the body such as the “third eye,” as we want the subtle energies of Om to be free to manifest themselves as is best at the moment.
10) Thoughts, impressions, memories, inner sensations, and suchlike may also arise during meditation. Be calmly aware of all these things in a detached and objective manner, but keep your attention centered in your intonations of Om in time with your breath. Do not let your attention become centered on or caught up in any inner or outer phenomena. Om can also produce peace, awareness and quiet joy in your mind as well as soothing radiations of energy in the physical and subtle bodies. Be calmly aware of all these things in a detached and objective manner-they are part of the transforming work of Om, and are perfectly all right - but keep your attention centered in your intonations of Om in time with your breath. Even though something feels very right or good when it occurs, it should not be forced or hung on to. The sum and substance of it all is this: It is not the experience we are after, but the effect.
12) Remember: Om Yoga meditation basically consists of three things: a) sitting with the eyes turned up and then closed; b) being aware of our breath as it moves in and out, and c) mentally intoning Om in time with the breathing and listening to those mental intonations - all in a relaxed and easeful manner, without strain.
13) At the end of your meditation time, keep on intoning Om in time with your breath as you go about your various activities. Since you cannot keep your eyes turned up outside meditation, as much as is possible or practical try to keep a general awareness of the “thousand-petalled lotus” of the brain all the time, feeling that the breath and Om are taking place there. In this way you can keep “near” the Chidakasha state you experience in meditation.
Om Yoga is simple and easy because it goes directly to the root of our bondage which is a single (and therefore simple) thing: loss of awareness.
The eyes have a definite esoteric effect on the mind and its subtle energies as well as the polarization of those energies. When the eyes are turned down, they lead into subconscious experiences, especially when they are closed, and even into the sleep state. When the eyes are held straight ahead, they keep us alert and aware of our surroundings, even if the eyes are closed, and confine our awareness to the ordinary conscious state. When the eyes are turned up, they begin transferring our awareness into the levels of superconsciousness. For when the eyes are turned up, the thousand-petalled lotus of the brain, the Sahasrara, begins to open and become active.
Meditating with upturned eyes causes the subtle mental energies that pervade the body to begin moving upward into the higher centers of perception in the brain and its astral and causal counterparts.
You will find that on occasion you may even experience that the subtle breath is taking place within the area of the eyebrows or the forehead.
it may help to think of looking upward into the sky
Anyone who understands the theory of mantra yoga and the relation of vibration with consciousness should be able to see that there is nothing inherently impossible in the idea of a mystic syllable possessing such a power.
We must meditate on the Self-not on external deities or symbolic forms of psychic states. As Sri Ma Sarada Devi said: “After attaining wisdom one sees that gods and deities are all maya.” (Maya=Matrix)
“When you utter 'Om' it travels not only all around the earth but throughout all space and eternity”-so said Paramhansa Yogananda. Thoughts do not cease the moment they pass from the conscious mind. They spread out around us, into our aura, the subtle field of biomagnetic and mental energies around our physical body, and then on into the surrounding creation, ultimately extending to the farthest reaches of the cosmos and then returning and striking back into our aura and mind. This is the process of mental karma. By continually doing repetition and meditation of Om, we set up a continuous current of spiritual vibration that in time becomes a perpetual inflow of higher consciousness as it returns to us after having extended throughout creation and benefited all things and all beings therein. In this way we create the highest form of spiritual karma, uplifting and divinizing both ourselves and all that exists.

A final word
All the theory and eulogy in the world regarding a meditation practice mean virtually nothing. But practice is everything. In meditation more than anything I know, practice certainly does Make Perfect. Meditation produces steady growth if there is steady practice. And the practice is so marvelously simple.
Om Yoga is the means by which we answer for ourselves the prayer:
Lead me from the unreal to the Real.
Lead me from darkness to the Light.
Lead me from death to Immortality.

http://www.atmajyoti.org/med_om_in_upanishads.asp

Om in the Upanishads, Bhagavad Gita, and Yoga Sutras
Swami Nirmalananda Giri
Here some excerpts:

Only a fraction of the world’s population is formally imprisoned, but the entire human race is imprisoned in the body and the earth itself. None are free from the inevitability of sickness, age, and death, however free of them they may be at the moment.

Our real self, the spirit, is ever perfect and free. But we have forgotten that.

Eternal Truth in its purity is found in twelve basic texts of the Upanishads and the Bhagavad Gita, being a digest and exposition of the upanishadic philosophy with emphasis on its practical application.

Although Om is to be found in each upanishad, at least as part of the opening and closing mantras of blessing, eight of them have sections dealing with Om Itself. Here some basic statements from the Upanishads.

Om is "the Primeval Being."

Only chant Om to obtain fulfillment of their desires-especially the desire for divine knowledge.
"Speech [vak] and life force [prana] are joined together in the syllable Om.

Prana means the Universal Life Force which is incarnate in every living being in the form of the five pranas (Sinne?) that support all the functions of living organisms. Om is more than sound, It is Life Itself.

Om is not only the seed of omniscience and omnipresence, It is also the seed of omnipotence.

Since our soul is always breathing Om, by intoning Om we put ourselves in tune with the very wellspring of our existence, linking up with our inmost consciousness.

Om lifts us beyond the sphere of cause and effect and sets us free.

Om is both transcendent and immanent. In which ever plane we wish to abide, Om is the basis, the illuminator, the key to comprehension and mastery.

From the original Sound, Om, all things have come into manifestation as Its extension-embodiments. Everything that has ever existed, now exists, or shall exist, is the expansion of Om. Om is all-embracing Eternity, containing and transcending past, present, and future. There is nothing but Om. That being true, the upanishad then says: "The Self [atman] is of the nature of the Syllable Om... Thus the Syllable Om is the very Self. He who knows It thus enters the Self [Supreme Spirit] with his self [individual spirit]." By means of Om, the eternal wave merges into the eternal Sea.


The Great Galactic Central Sun and Death and Liberation

The upanishadic sages had much to say about the great galactic central sun as the source of life.

The great galactic central sun continually vibrates Om according to the upanishads and those who continually intone Om align themselves with its evolutionary energies and greatly quicken their upward development. Having established a profound affinity with the sun through a lifetime of Om meditation, they easily pass upward and through the sun, escaping the compulsion to material rebirth.

the rays of the sun go to both these worlds, this one and that yonder. They start from the yonder sun and enter into the nadis. They start from the nadis and enter into the yonder sun... When a man departs from this body, then he goes upwards by these very rays or he goes up with the thought of Om. As his mind is failing, he goes to the sun. That, verily, is the gateway of the world, an entering in for the knowers

Rebirth occurs because we have not fully realized our divine potential. Om unfolds that potential and thereby frees us.

As milk becomes diluted in water, so the consciousness of the departing soul becomes dispersed and wanders, confused. But this is not so for those who cling even in death to the repetition of Om

The Pranava Manthra "OM" is pronounced as "AUM" with a long vowel sound. "OM" is the comprehensive Universal, holy Sound Symbol and Signifier of the Supreme Infinite Reality. The Universe is said to have been created with the Supreme Reality manifesting first as the Ether or Space of the Pancha Bootham [Five primary Elements] and comes in the form of "Sound" - "Om". It manifests as the totality of existence on four Cosmic and Spiritual planes; Physical, Psychical, Causal or potential and transcendental. Thus OM represents the Supreme Reality and all His manifestations and creations. Sri Ganesha, as God manifesting and represented with the face like an elephant is symbolic of the shape Om and is called "Pranava Swaroopa". " All this Cosmic Universe is the Eternal word Om. Its further explanation is this: the past, the present, the future and everything is just Om. - And whatever transcends the three divisions of time -- that too is just Om. .. -- .. Thus the Word Om is verily the Self [Aathman]. He who knows this, with his self [Jiva], enters the Self [Aathman]. -- [ Maandukya Upanishad ]

" What is Om, O' Sathyakaama, is verily the Higher or Transcedental Brahmam and the Relative or Personal Creator-God Brahmaa. Therefore the Knower attains either of the two by this means."

" ... Verily, with this Om-kara as the Support, the knower attains what is peaceful, undecaying, immortal, fearless and Supreme - Aathman / Brahmam." -- [ Prasna Upanishad. ] " Pranava - the Mystic sound Om - is the Bow; the soul within, the arrow; and Brahmam, the target. One should hit that mark with an undistracted mind, and like the arrow, become one with It."-- [Mundaka Upanishad.]

" All this Cosmic Universe is the Eternal word Om. Its further explanatyion is this: the past, the present, the future and everything is just Om. - And whatever transcends the three divisions of time -- that too is just Om. .. -- .. Thus the Word Om is verily the Self [Aathman]. He who knows this, with his self [Jiva], enters the Self [Aathman]. -- [ Maandukya Upanishad ]
--- Excerpts from: "Om Gayatri and Sandhya" - by Swami Mukhyananda...

www.authorama.com/god-idea-of-the-ancients-5.html
Om as Goddess
We have seen that according to various writers Om or Amm was the holy one whose name in India it was a sacrilege to pronounce. It was the eternal sun, or the Great Mother. As this word stands also for "tribe or people,” it seems to mean, too, that which binds, holds, or endures.
As Om or Amm signifies the Great Mother, so An or On means the Great Father. Concerning the word Amm-On, therefore:
"The association of the words signifying mother and father indicates that it is to such conjunction we must refer creative power

Am-Un, Am-On, An-Ubis, Om-An, An-K, An-U are all words containing one or more of the various syllables.