http://espn.go.com/nba/tradeMachine?tradeId=77l559mCeltics do this because: It's a great way for us to get a good point guard, and a VERY good shooting guard for the future.
James Harden is a potential all star and OJ Mayo could be a very solid starting point guard. His position in Memphis is shooting guard but in college he was a top 3 point guard in the nation. Tony Allen starting at the 2 spot means they need OJ's scoring off the bench and he's kind of been force-moulded to play the position as an undersized scoring wing player.
OJ comes to Boston and gets to play his preferred point guard spot and we have a very good scoring guard combo punch of OJ+Harden.
OKC does this because they have always talked about Westbrook not passing enough and taking shots from Durant.
They can shift Westbrook to the 2 spot, Durant to the 3, and have Rondo getting everyone the ball.
This is great for OKC because Rondo's ability to get Perkins and Ibaka the ball on penetration means they get more offensive production out of their big men, it also means that Rondo is the ball captain, not Westbrook, so Durant gets even more of the ball which is only a good thing.
Westbrook and Rondo and Durant running the floor together would be amazing to watch.
They can bring Sefalosha off the bench to replace Harden as the 6th man.
Memphis does this because they get a first round pick, a solid back up center, and a potential replacement for OJ in Etwaun Moore. Memphis is least likely to make this deal but if you throw in the first round pick I think they take it.
(correct me if I'm wrong but OJ can walk at the end of the season anyway).
Celtics receive:OJ Mayo
James Harden
OKC receives:Rajon Rondo
Greg Steimsma
Memphis receives:Etwaun Moore
Cole Aldrich
Royal Ivery
Boston Celtics first round pick 2012.
This leaves us with a starting five of:
PG:OJ Mayo
SG:Harden
SF:Pierce
PF:KG/Bass
C: ? Free agent or a fill in until a top dollar center is available.
Get Jeff Green, Ray, KG coming off the bench with Johnson, Bradley and Pietrus and we are looking pretty good once we get a solid center in Dwight Howard

I think OJ would be at his best at the point guard position, he can become a Westbrook type PG but is a better passer/facilitator whislt being able to hit the open jumpshots that Rondo struggles with.
Don't crucify me please....