O'Bryant has some scoring skills and can block shots. I would like to see him work harder and earn some more PT in the 2nd half of the season and possibly displace BBD from the lineup. I don't know, I am not so high on BBD - he is too short to be a true center and too slow to be a true power forward. So I might be shopping Glen Davis this February if I got paid to be Celtics GM.
Now for signing up Mutombo, I would be careful since he has been in this league forever, it seems. I just don't think he has much left. It would be like that Artis Gilmore signing back in 1990.