According to the NCAA Complex Invasion College Football Production Model, Oklahoma currently has the #97 defense in the FBS, which is actually ahead of Oklahoma State's defensive ranking (#112). From the prior week, Oklahoma's defensive ranking dropped from #51 (above average) to below average. That is more of a problem, since until the Texas game, Oklahoma has played teams that are all below average in overall performance. Thus the first real test for Oklahoma's defense resulted in a loss.
But this is not new for Oklahoma. Since Mike Stoops arrived at Oklahoma, the Sooners' have had only two seasons where their final defensive ranking was above average - 2013 and 2015. Other than that, the Sooners' have been below average in terms of defense. In fact, the Sooners' finished the 2017 season lower ranked in defensive productivity than they currently are ranked.
While the defense is currently below average, the real concern is that is it really below the current rankings of the teams that are in the running for the FBS Playoffs: Georgia (#3); Washington (#4); West Virginia (#6); Alabama (#10); Clemson (#11); Notre Dame (#15); UCF (#25). Notice that Ohio State is not on this list as they are currently ranked above Oklahoma, but are below average.
On the bright side, Oklahoma currently has a top 10 offense, so hopefully the Sooners' can outscore their opponents.