Ugh, here let me make it clear. Yahoo ranked Kristic at 22, Jeff Green at 25. Thats not a good frontcourt! Clearly I'm not the only one who thinks they are below average.
The problem I'm having is that you are using words like "good."
Ibaka is a very good rebounder and very good defender. Jeff Green is well above average too.
Look at the options OKC has to use at 4-5:
A) Krstic+Ibaka - solid at defending the KG-Perkins Bynum-Gasol type of front-courts. Not a very good low-post presense (unless they are playing against teams like GSW) but they will get you garbage points in the paint, regardless of who they are playing against. And at a high percentage.
B) Collison+Ibaka - Niether is a true NBA Center and niether are very good established low-post scorers. But that is a very good defensive and exceptional rebounding front-court.
C) Krstic+Green - Green simply cant defend frontcourts like LA or Boston. But does a decent job on guys like Nowitzki. This isn't a very good rebounding group when you see them together at the same time. Usually they use this group when they want to gamble on defense and run the fast break. Green is a great open court scorer. And can hit the 3 ball in transition at the 4 position.
D) Ibaka+Green - This group is even better at forcing turnovers and running the break. Offensively it is very good at spreading the sloor in the half court. Green can also defend most NBA PFs pretty exceptionally.
E) Kristic+Durant - pretty good rebounding front-court because both of those guys have height. Durant won't typically play much PF but he is surely capable on offense. He isn't the type to defend a Gasol or Duncan on defense though. This would mean great shooting in the half-court offense.
F) Ibaka+Durant - again, pretty good rebounding (unless you are playing LA, MEM, or CHI). And stellar in a transition offensive, high-paced style. These guys are both very solid post defenders.
I didn't even use Collison much because he is designed to be used for limited minutes. He gives you solid defense at the PF position, but crappy at rebounding or defending against typical NBA Centers. But he can score a little bit in the post.
Nor did I use Cole Adrich at all. He is more like a wild card.