I don't see OKC as any competition for GSW. OKC are top heavy and have failed to gel as a team. They are not a threat to GSW. GSW will smoke them in a playoff series. 5 games.
Even without Roberson (and Brewer will help them), OKC is a horrible match-up for the Warriors because they can win the PG battle and they can make Durant work on both ends of the floor like very few teams. There is a reason that OKC has beaten the Warriors by 17 and 20 (they did get crushed in the 3rd and most recent game, though PG and RW were a combined 5 of 29 and CA wasn't a much better 6 of 17). The Thunder are definitely thin, but in the playoffs when you can realistically play 8 or 9 man rotations, that thinness is less important (Adams, Anthony, George, Huestis, Westbrook, Patterson, Grant, Felton, and Brewer is a pretty solid 9). They have the right mix of scoring, defense, rebounding, etc. to really push the Warriors and make them work. And more importantly Westbrook isn't afraid of that team. He will physically brutalize Curry on both ends of the floor. George seems to relish the Durant match-up as well, and if they put Green on George, then Durant still has to guard Anthony so he can't just hide on that end of the floor. Klay is the key to that series. If he is on, then GS can certainly win in 4 or 5 (though they will be physically tough games), but if he is off a bit, I'd expect that series to be very close and very tough for the Warriors.