According to TAITREYA Upanishad the origin of language is assigned to PRAJAPATI, from whose meditation on the three words arose the three Vedas and from his meditation arose the three syllables, BHOOR, BHUVA and SUAH, which represent earth, atmosphere and sky. From his meditation on these three originated the divine syllable of OM, which coordinated all speech and represented the totality of the world. The syllable OM also represents the TRIMURTI (triad) of Brahma, Vishnu and Shiva.
Shiva's drum produced this sound and through it came the notes of the octave, (i.e.) SA, RI, GA, MA, PA, DHA, NI. Thus by this sound Shiva creates and recreates the universe. OM is also the sound form of Atman.

The Upanishads state that everything, existent and non- existent, can be grasped by uttering the sacred syllable of OM. The psychotherapeutic efficacy of OM is deemed limitless and its utterance redeems all errors in the performance of a sacrifice. Meditation on OM satisfies every need and ultimately leads to liberation. Nearly all the prayers and recitals of sacred passages are prefixed by the utterance of OM. Its equivalent is OMKAR, venerated in the same manner and is thought to be the representation of God Himself.
Musically, it is also held that the term OM or AUM is made up by three base notes ' A ' 'U' 'M' or the basic 'SA' 'PA' of the fundamental scale and again Sa (the base note) of the immediately higher scale. When one pronounces these notes in continuity, all the basic notes from Sa to Ni also sound. Similarly when one pronounces AUM correctly, all the basic sounds also echo. It is believed to be the traditional way of clearing all the impediments in the vocal chord to make one chant the hymns correctly. Their unison makes one not only sound sonorous but also acts as the necessary preparation to chant a Mantra (Incantation) correctly. It is for this reason that all the Vedic Mantras has 'OM' or' AUM' as the first term.
Readers of the Bible are familiar with Amen and the word of God. They probably have heard of OM too. What might be new is that all these words have, in fact, the same meaning and also the same origins. Physicists know that all of matter, our whole Universe, is made of vibrational energy. Atoms are no longer pictured as solid balls orbiting a solid center or proton. Electrons and protons are now explained as energy concentrations with a certain vibrational pattern. This is the vibration which is called by readers of the Bible Amen or the word of God. Hindus call it OM or AUM. Moslems call it Amin. Since OM is the original vibration, all other mantras are derived from this one Master Mantra. Usually when we hear OM reproduced on TV or at a spiritual gathering it is pronounced like dome without the 'd'. However, OM can indeed be perceived during meditation but sounds more like the o-sound in ball without the 'b'. It is a deep roaring sound and much closer to the Tibetan mountain trumpets which are used to reproduce the sound and remind people that it might be time to return Home.
Japa
Repetition of a name of God may be used as a mantra and is then called japa. A mala is usually used for automatic counting but, when used in connection with japa, it is more for anchoring the mind and make it more rhythmic. Japa and mala are for a similar purpose as the stick given to an elephant in India when lead through narrow streets where merchants display their goods on tables. Without carrying the stick in his trunk, the elephant will move its trunk to the left and right knocking off the display, but, when given the stick the elephant will carry the stick straight ahead. In order to keep our trunk - the mind - still, we are practicing meditation, japa, or any other spiritual technique. One traditional name for the practice of japa is Rama. However, Christ or any other name of God is fine as long as the association is there. If the association is not there, any other mantra may produce the same result. Japa should be repeated constantly, this technique, more than any other, is the exact equivalent to the first commandment of the Bible: "You shall love God with all your heart, mind and soul." What else would it be if we were to concentrate fully on God - with all our heart, mind and soul?
Bija
There are also bija-mantras which may be seen as keys with which one might address a certain chakra (energy center in the spine and brain) for example. The main keys to the seven chakras are Lam, Vam, Ram, Yam, Ham, Ksam, Bam (or OM). Each chakra addressed by one of the major mantras also includes other bijas.

Mantra Yog
The word mantra is related to manas (the mind). A mantra is a sound or word. It is generally used to help focus the mind on a single thought until thoughtless Beingness is established. As a tool to achieve Stillness, the mantra is to be discarded at the moment Stillness is achieved.
Yoga means to unite with our Origin. Mantra Yoga would then mean to unite using the tool of mantra. This process is called Concentration and Meditation.

Mantras are sometimes also applied to change circumstances, to regain a healthy condition for example. Those mantras are specially formulated to carry a certain vibrational power. For such purposes it is important that the proper pronunciation is also imparted.