That's either a lack of physical conditioning by your line, a lack of mental conditioning by your QB, or poor coaching. Which is it?
My sense is that it is poor coaching and flaws in your offense gaming structure. Your team lacks depth in the defensive area, so as the game goes on and the teams are wearing each other out, the offense is dialed down to give your defense a chance to rest. This in turn effects offensive production because your system is based less on exploiting talent mismatches, but on tempo and taking advantage of defense misalignment and lack of substitution caused by quick snap. When things slow down, the mistakes are corrected and they get the right person in and your QB isn't quite as efficient. It isnt that your players are good, it is what they are doing and how they do it that is good. When that changes in the fourth quarter, you lose your advantage, your production, and sometimes the game. OSU doesnt have top 10 talent but has the ability to play at a Top 10 level under certain circumstnaces. This is why people coast to coast get annoyed when OSU fans insist the Aggies are a Top 10 program.
And that's why I think it will be a shoot out with the team with the least mistakes will win. Every major program Tulsa has played during the Malzahn era and especially the Montgomery era has stated that they play an up tempo no huddle style but they were not adequately prepared for the tempo of Tulsa. Stoops always bitched about it. Urban Meyer made comments in his presser last year that he feared the game in Columbus might get out of control if the defensive coaches and players didnt adapt to what Tulsa was doing. They didnt, but the weather changed the game in their favor. In short, OSU has to run tempo for their offense to work. We will run any program into the ground, not matter how athletically gifted or conditioned. At a certain point, your team will break. The only question is whether the score will be close enough at that point for it to make a difference. If TU takes care of the ball and continues to be able to get 7 and 8 yards a rush against playoff caliber defenses like last year, then OSU had better worry.