From http://shivamantras.com/ :
OM is the absolute reality. It is said to be “Adi Anadi”, without beginning or the end and embracing all that exists. Om is beyond any limit, transcendental, and indestructible. OM represents the wholeness of the universe and the eternal vibratory sound of brahman (the universal soul) that permeates it. Indeed, Om is called shabdabrahman, the sound-form of brahman. Om is a mystical or sacred syllable. The syllable is taken to consist of three phonemes, a, u and m, variously symbolizing the Three Vedas or the Hindu Trimurti or three stages in life (birth, life and death ). A-U-M represents the divine energy (Shakti) united in its three elementary aspects: Bhrahma Shakti (creation), Vishnu Shakti (preservation) and Shiva Shakti (liberation).In upanishads and sutra:Om consists of the three sounds a (a-kara), u (u-kara), m (ma-kara). A-kara means form or shape like earth, trees, or any other object. U-kara means formless or shapeless like water, air or fire. Ma-kara means neither shape nor shapeless (but still exists) like the dark matter in the Universe. When we combine all three syllables we get AUM which is a combination of A-kara, U-kara, and Ma-kara.According to the Mandukya UpanishadOm is both atman and brahman: it is the past, the present and the future, as well as all that transcends time.In PuranaAum or Om is the mystic name for the Hindu Trimurti, and represents the union of the three gods, viz. a for Brahma, u for Vishnu and m for Mahadev which is another name of Shiva. The three sounds also symbolize the three Vedas (Rigveda, Samaveda, Yajurveda).Om is the mantra of assent. It means yes and affirms and energizes whatever we say after it. That is why all mantras begin with OM. OM is also the mantra of ascent and causes our energy to rise upward into the infinite. By reciting Om, a person gains access to the powers of the universe; and meditation on Om is said to lead to enlightenment and immortality. OM is expansive and increases the fire, air and ether elements, particularly ether. It also gives strength, protection and grace.