The Spurs offense worked both because of floor spacing and because the ball carrier always had at least two options. As long as he made the right decision with the ball, which a group full of veterans like the Spurs are wont to do, the offense would produce a clean look, usually a corner three. The basic setup for the play looked like this:
The screener was typically Tim Duncan, but always a big man, and the ball carrier was usually Tony Parker. First, take note of the floor spacing. The Spurs have two shooters in the short corners and a third shooter on one side of the floor. The pick and roll was almost always directed toward the side of the floor with two players, about which more later. Once Parker would get over the screen, Duncan would roll to the basket:
At this point, Parker has two options: drop the ball to Duncan who is cutting toward the basket or take the ball to the rim himself. This is where the Thunder's defensive adjustments (or lack thereof) failed miserably. They refused to switch on the pick and roll (not a horrible idea), but the on-ball defender (we'll say Russell Westbrook) always went over the screen, meaning he was immediately in a trailing position. If the man defending Duncan shows on Parker, then Parker dumps the ball to Duncan rolling to the basket. If Duncan's defender stays with him, Parker attacks with Westbrook already on his back.
The real problem is that the Thunder didn't have a defensive strategy for this, which usually meant that both options were open. Toward the end of the game, they started to trap Parker high, but that didn't work particularly well because Duncan would just slip the screen and the trap was half-hearted at best. The real brilliance of this offensive design doesn't become apparent until Parker makes the slip pass to Duncan though:
As I mentioned before, the pick and roll always went toward the side of the floor with two Spur players. Once Parker dumps the ball off to Duncan, he has a relatively clear lane to the basket. His defender had to slow Parker from driving into the lane and is now in a trailing position on Duncan. This requires help defense. Since Parker originally drove toward the side of the floor with two Spurs shooters, after Duncan slips the screen, he's running down the backside with only one Thunder defender who has a choice to make: come off his man in the corner to stop the drive or else concede an open layup. Now you can see the two options Duncan has: drive to the lane if the help defense remains on the corner shooter or kick the ball out if they pack the lane.
That's it. And I mean, the whole game. Watch the first three plays here. Despite the fact that Westbrook gets the block on the first, they are all the exact same play:
The Spurs ran that play all game and the Thunder never adjusted to it. I don't know that I've ever seen a team so hopelessly lost against a single play. You go to a zone defense. You play ball denial on Parker all the way down the court (you'd rather have him taking a corner three than Ginobili, for example). You trap hard on the pick and roll. You do anything. Scott Brooks watched as the Spurs turned into the Harlem Globetrotters against his incredibly young and talented squad.
If the Thunder are going to have any chance in this series, they're going to need to figure out how to defend this single, basic play. Fortunately for them, they have plenty of game film to correct their problems.