ISU and oSu are weird animals right now. ISU in particular. There's no reason for them to have taken the marked step back this season that they have. Their game relies on force rather than finesse, but they still have their QB, RB, and TE that saw them dominate opponents last season. They're not awful, but rather just not quite as in sync as they were.
oSu is obviously missing their weapons. Their QB was always more of a function of his RB and WR than raw personal talent. This season thus far he's trying to force things. Some of his throws are mind bogglingly bad in which he just tosses a pass directly to the defense. The running game just isn't setting them up for success.
Really, the Big 12 right now is OU not able to ply their game due to a running game weak between the tackles, but still winning games. After them comes a Texas team with real promise, but no history. They pass the eye test, but are obviously still getting their sea legs. Behind these two are basically 7 teams that are good but not great. The statistical likelihood is that they'll cannibalize each other depending on who gets the edge that particular Saturday. Them, then KU.
OU and Texas aren't so dominant as to escape the pack. One or both will likely post a bad loss to them at some point.