Om is a mantra. It is believed that chanting OM will invoke a perspective of yourself as a part of the universe. In Sanskrit, OM is spelled Aum. Each letter of the word has a separate meaning.

a represents the self in the material world
u represents the psychic realm
m represents the spiritual light within the self

It is usually chanted during meditation or yoga practice. Sit and quiet yourself, decompress so to speak. Focus on your breathing and when you are ready to chant, take a deep breath in. On the exhale, say OM. Make the sound as long as your exhale, stretch the "au" sound out for most of the exhale and then move into the "mm" sound. Let the sound fill the room, fill your being. Inhale deeply again, and once again chant OM. Chant as many times as you wish, I usually chant three times after yoga practice. Some people follow OM with shanti shanti shanti.