I think Oklahoma City can win this series, but they need a lot of things to break their way.
A) Westbrook really needs to play the best series of his life, particularly on the defensive end. Even when OKC was playing well against San Antonio, I felt that Westbrook still struggled getting around screens and allowed Parker to penetrate into the paint and finish at the rim. Parker will hurt you if he does that, but Curry will MURDER you if they are that non-committal on defense.
B) OKC swung the series by riding with Kanter and Adams on the floor at the same time. I'm not sure they can get away with playing them both at the same time against GS. If GS goes with their traditional lineup, I assume Kanter is going to guard Green, which is troublesome because those Curry/Green pick-and-rolls force the man guarding Green to leap-out and attack Curry or else Curry is going to score 50 points and make 10 three pointers. Adams can probably do that but Kanter? Maybe Kanter guards Iguodala and Durant guards Green but I'm not sure that is much better. If Kanter plays the best defense of his career and is able to hang in there defensively, than OKC has a real shot because they can bury GS on the boards and pound the offensive glass.