From the Encyclopedia Britannica online:
Om, in Hinduism and other religions chiefly of India, a sacred syllable that is considered to be the greatest of all the mantras, or sacred formulas. The syllable Om is composed of the three sounds a-u-m (in Sanskrit, the vowels a and u coalesce to become o), which represent several important triads: the three worlds of earth, atmosphere, and heaven; the three major Hindu gods, Brahma, Vishnu, and Shiva; and the three sacred Vedic scriptures, Rigveda, Yajurveda, and Samaveda.
Thus Om mystically embodies the essence of the entire universe. It is uttered at the beginning and end of Hindu prayers, chants, and meditation and is freely used in Buddhist and Jain ritual also. From the 6th century, the written symbol designating the sound is used to mark the beginning of a text in a manuscript or an inscription.
Om, in Hinduism and other religions chiefly of India, a sacred syllable that is considered to be the greatest of all the mantras, or sacred formulas. The syllable Om is composed of the three sounds a-u-m (in Sanskrit, the vowels a and u coalesce to become o), which represent several important triads: the three worlds of earth, atmosphere, and heaven; the three major Hindu gods, Brahma, Vishnu, and Shiva; and the three sacred Vedic scriptures, Rigveda, Yajurveda, and Samaveda.
Thus Om mystically embodies the essence of the entire universe. It is uttered at the beginning and end of Hindu prayers, chants, and meditation and is freely used in Buddhist and Jain ritual also. From the 6th century, the written symbol designating the sound is used to mark the beginning of a text in a manuscript or an inscription.
And... an interesting article in Huffington Post:
OM Alone
