No, 'Om' isn't a Buddhist teaching in itself.
Even in Tibetan Buddhism, it usually appears at the beginning of Tibetan Buddhist mantras which are repeated alone or as part of a deity practice...eg Om Mani Padme Hung. (sanskrit) Om isn't recited by itself.
I am a Theravadin Buddhist, so I don't use mantras or meditational deities like in Tibetan Buddhism.
Here's some info about Om Mani Padme Hung
http://www.dharma-haven.org/tibetan/meaning-of-om-mani-padme-hung.htm