Why?
109th in total yards at 5388 (ISU had 3933)
111th in yards per game at 449 (ISU had 358)
127th in passing yards at 3439 (ISU had 2524)
64th in rushing yards at 1949 (ISU had 1409)
100th in points per game at 32.8 (ISU had 22.4)
Keep in mind that there are 130 teams in FBS. They were in the lower half of all but one of those categories, and in passing yards they were the 4th worst in all of FBS.
That is why people bag on OU's defense. They didn't win games because of their defense; they won games because their opponents couldn't keep pace putting up points.
I understand that, but you also have to look at other things. OU's offense is one of the most EFFICIENT in the country. They score fast and that forces their defense to be on the field more. For example in the WV game the OU offense ran 65 offensive plays and WV ran 90.