Wherever Jermaine does the least amount of damage to the Celtics offense is where he should play.
That means wherever he'll be asked to create the least amount of his own offense (post ups). Likely with the starters.
Limit the damage as much as possible.
Remember, he's replacing Perk. Are you expecting the switch from Perk to JO to damage our offense?
When Jermaine O'Neal plays off of others and finishes plays he is an effective offensive player. Someone who will score at a solid clip and with good-to-very good efficiency. Very good finisher around the rim and has a serviceable jump shot out to about 15-16 feet.
When Jermaine O'Neal is asked to create his own offense he becomes the most ineffective offensive player in the league. He is a poor passer, he spends too long on the ball and consequently creates stagnant offenses. In his one-on-one offense he also scores at a horribly inefficient rate. The combination of those two large negative contributions has a large negative effect on his teams offense. It's been this way for three years now.
The key to Jermaine O'Neal being a useful player versus a net negative is avoiding him creating his own offense. When he creates his own offense he hurts his own team. When Jermaine is asked to create offensively he negates his positives contributions (defense, rebounding, complementary offense).
----------------------------------------------
On this Celtics team, I have no confidence whatsoever about Jermaine being given and/or accepting his role.
After watching Glen Davis take advantage of a fairly equal opportunity offense and try to create his own offense last season + how desperate this team is for a low post presence with KG refusing to do so and Sheed retiring ... I am terrified of the role Jermaine O'Neal is going to have offensively for this team next season.
------------------------------------------------
Jermaine O'Neal is a much better fit alongside a ball dominating player like a Dwyane Wade or a Chris Paul. Someone who will spoon feed him like a child. Not let him screw up the team's offense by pretending he is still 25 and a talented go-to scorer.