http://www.breakoutofthebox.com/OmSymbol.jpg

You might find a Sanskrit font here:

http://salrc.uchicago.edu/resources/fonts/available/sanskrit/

Most likely, you have Arial Unicode MS installed in your operating system.

More information: http://en.wikipedia.org/wiki/Om

(Name, phonology and written representation section).

If it's only for one letter, it's easier to use a bitmap (or vectorial) image.