Robert Marcato's mother died last week and has now moved into the Osbournes' family home in Los Angeles.
The Osbournes will pay for the teenager's college education and he will even feature in the next series of their hit reality TV show.
Sharon said: "Robert will be part of the show as 'Baby Osbourne' - even though he's 18."
The second series of The Osbournes is to be broadcast in the US this Autumn.
The first series got 10 million viewers per episode.