Rajon Rondo+ Marcus Smart+ Chris Johnson, '18 Nets pick, and any Celtics pick for Westbrook+fillers, and OKC '16 first round pick, 2nd rounder.
Why OKC does it:
Gives them Rajon Rondo who can still orchestrate a great transition offense. Yes Rajon Rondo pumps the balls a lot, but he was heavily known as the one man fast break during the playoffs. Getting someone with playoff experience, and who specializes in giving teammates the extra boost in points, and touches. I believe Durant would be conserve more energy shooting from easy passes rather than exert too much energy in creating his own. Rondo is also a great team help defender, individually, Rondo isn't one of the best, but he can help pick off steals, and inaccurate passes.
Marcus Smart while unknown provides them with a strong pit bull on defense, and is a great transitional guard too. OKC is known for running on the fast break, focusing on defense, and ball movement spreading for shooters.
A lineup of Rondo, Smart, Durant, Ibaka, Perkins is a pretty tough defensive duo that can drive inside, protect the paint, and Durant and possibly Smart (?) can get to the free throw line at least 5-6 times a game.
I know people say Westbrook helps create double teams, etc. I firmly believe Rondo can still do almost the same things Westbrook can, but his passing is what can get Ibaka, who is great at mid range shots, and only improving, and Durant the necessary passes needed.
Why Celtics does it:
Gives us a young point guard/scoring guard that can be our superstar. Westbrook despite his warts is still a top 5-10 player. Bringing him in could possibly give us a go to guy, and someone who can get to the free throw, create shots for others, and use his own athleticism to get buckets.
Once Westbrook comes here, its his team, and that is undisputed. With Bradley here, it also allows him to defend like Rondo, while using his ability to pick off passes with his speed to create havoc on the fast break.