The spurs are really balling right now. There's no really good game plan I can think of defensively if you're OKC. If you're OKC the only thing you can try to do is disrupt the rhythm and timing of their offense as much as possible. I would run some 3/4 and 1/2 court trap defense, not to get a steal, but to try to make them setup later in the shot clock. I would get as physical as the refs allow and take some fouls to get through and around their screens. Also I would go against conventional wisdom and actually minimize my doubling of Duncan and try instead to keep their supporting cast bottled up and let Duncan go one-on-one against Perkins/Nazr/Collison. I would use their 18 fouls to put Duncan on the FT line everytime he's about to dunk. Duncan is going to get his regardless, so I would actually make him go for 40+ every night over a 7-game series and see if he can still carry the load every night on those 36-year old legs.
Offensively, I would push the tempo and try to get easy fast-break points. I would go small for about 10-15 minutes each game and put Durant at the 4 along with Thabo/Harden/Westbrook, and get one of the San antonio bigs (splitter, diaw, bonner) off the floor. Finally on offense, I would be preaching attack-attack-attack. They need to try to get into the lane on every possession. OKC is a great FT-shooting team. They cannot settle for jumpers (I'm talking especially to you westbrook) in this series even if they're open, as the spurs will give them room and bait them into taking them. They need to get in the lane, draw fouls as much as possible (maybe even get duncan in foul trouble), and get to the foul line.