…Oṁkāra, or the praṇava, is the seed of transcendental realization, and it is composed of the three transcendental letters a-u-m. By its chanting by the mind, in conjunction with the breathing process, which is a transcendental but mechanical way of getting into trance, as devised by the experience of great mystics, one is able to bring the mind, which is materially absorbed, under control.
Oṁkāra is the seed of all transcendental sound and it is only the transcendental sound which can bring about the desired change of the mind and the senses. Mind or desire cannot be stopped, but to develop a desire to function for spiritual realization, the quality of engagement by the mind has to be changed.
In the Bhagavad-gītā, the praṇava (oṁkāra) has been accepted as the direct, literal representation of the Supreme Absolute Truth. One who is not able to chant directly the holy name of the Lord, as recommended above, can easily chant the praṇava (oṁkāra). This oṁkāra is a note of address, such as “O my Lord,” just as oṁ hari om means “O my Lord, the Supreme Personality of Godhead.” As we have explained before, the Lord’s holy name is identical with the Lord Himself. So also is oṁkāra. But persons who are unable to realize the transcendental personal form or name of the Lord on account of their imperfect senses (in other words, the neophytes) are trained to the practice of self-realization by this mechanical process of regulating the breathing function and simultaneously repeating the praṇava (oṁkāra) within the mind.
The Hare Krishna Movement 