While it seems that mostly everyone has been focusing on the lack of offense (specifically from Danny Green, Gary Neal, and Matt Bonner, not to mention the turnovers), and that defense has never been a total strength of this team, I think it's worth mentioning that the Spurs defense has been poor most of the past three games. Part of this is to be expected- the Thunder have one of the league's best offenses, and it's hitting on all cylinders. But seriously. The Spurs are better than this.
Manu Ginobili is now starting in place of our second best wing defender, Danny Green. Danny played 15 minutes in game one, and only 3 minutes in game 5. Many have said that Gary Neal absorbed his minutes, but that's not true. Neal played only 15 minutes in game 5. Manu, Stephen Jackson, and Kawhi Leonard ate most of Green's minutes, and that's why I don't think the defense experienced a major drop off after the Manu/Green switch.
One of our problems defensively has been that Oklahoma City's bigs (or at last Serge Ibaka) have been hitting jump shots. Ibaka is a solid mid range shooter, has been all season, and so this shouldn't be a surprise to anyone. It's still preferable to have Ibaka shooting jumpers than Durant or Westbrook, however I think a simple adjustment would be to put Boris Diaw on him instead of Tim Duncan. Offensively, one of our strategies was to draw Ibaka away from the basket to negate his shot blocking. When we've got Duncan covering Ibaka, they do the same thing to us. If Duncan is on Perkins, then the only way the Thunder pick and roll can hurt us is if he rolls hard; if he pops, he's not a threat like Ibaka is, and if he rolls slow, he's an easy cover. We've known now for a few years that Duncan's diminished lateral quickness can be exploited in the high pick and roll, and the Thunder are starting to do to us what the Phoenix Suns did. Diaw's quickness is wasted against Perkins, and Duncan's shot blocking ability is wasted covering Ibaka jump shots. It's a simple switch that should yield at least a marginal improvement to our defense.
It should also be obvious by now that the refs are not going to call many screen violations on either team. So, Manu and Tony need to stop hitting the deck every time Perkins sets a hard screen. It's not a total flop. I've run full speed into a brick wall before. It knocked me on my ass. Tony and Manu though have better balance, and most of the time could stay on their feet. The foul is not going to be called. Fight through it or go around it. If it's Westbrook, go under it, and encourage him to shoot as much as he wants. I'd like to see some traps anytime Perkins is the screener and either Durant or Harden (when he's going right) are the ballhandlers. Perkins is a horrible decision maker once he gets the ball away from the basket. If he catches the ball at the top of the key, there are four possible outcomes: he gets called for a travel/charge (35%* chance), he shoots a jump shot (25%*), he holds the ball and resets the offense (25%*), or he makes a good pass resulting in an open look (15%*). The first three are all defensive wins for the Spurs.
*all of those percentages are completely arbitrary, but in the small sample size I've seen, I believe them to be reasonably accurate.
Finally, any combination of at least two of Jackson/Leonard/Green need to be on the floor whenever Harden joins Durant and Westbrook. If that means playing small, play small. I think that over the course of the series, we can probably agree that Leonard and Jackson have been two of our best three players. What that says about our chances going forward, I don't know, but the way they are playing they deserve all the minutes they can handle at whatever position Pop can fit them in at.
As far as offensive adjustments, I don't see many that could be made, aside from better execution. Obviously, that's needed on both ends of the court, but limiting the turnovers will also go a long way to improving our defense.