As I said in another thread, I don't believe OKC is completely out of the title hunt THIS YEAR. Kevin Martin is a 18ppg scorer over his career, and will fill the scoring shoes of Harden nicely. Both Harden and KMart are below average defenders, so no real loss there, the biggest issue is Harden's ability to create offense - but with Maynor returning from injury this season, I don't think it'll be a back breaking issue for them.
When you factor in OKC's superior bench: Nick Collison, Thabo Sefolosha, Maynor, PJIII, and the potential in Jeremy Lamb, I think they match up well with the Lakers or any other Western Conference contender.