Om or AUM has a spiritual, a meditational and a mystical sound. It is of Sanskrit origin, highly sacred and important mainly in Hinduism. There is reference in Buddhism and Jainism.
It is mentioned as omkara literally "om syllable. When pronounced it also indicates the Beginning and End. It is the most important syllable for hindu meditation, which is done in ambrosial hours before dawn and finishing at sunrise. It can be pronounced softly as well as loudly and certain mastery has to be done by practitioner to say this at length. It has an open and a nasalised sound in itself.
Om is entire Cosmos. Om is mentioned in all the Holy Scriptures and books of Hinduism. Om can be seen virtually all over India.