it's the same thing as awesome, the Musso brothers (Mitchel, Mason and Marc) frequently use this. many of their fans now use this term more than the normal spelling.
@mitchelmusso Who says Ossum all the time?
hayley: I DO! mitchel you're so OSSUM! :D
-duuude, have you seen the new metro station music video?
-yeaaaah, it was ossum!(: