It's amazing how, in college ball, players who couldn't do much by themselves can assemble to be greater than their sum.
CeeDee Lamb is that player for OU's offense. Jalen Hurts is an adequate QB by himself. However, without Lamb (and to be fair, without Calceterra and about 4 starting RBs and an OT), he becomes a pedestrian running threat who turns the ball over more often than is acceptable. However, any time Lamb is on the field, OU's offense becomes capable of an immediate TD because he's almost impossible to tackle. He pulls the defense far enough out of sync that running lanes for the QB opens up.
OU absolutely has a good defense. LSU's offense will look a lot like Texas' did. They have seen it before (but LSU's is obviously better).
This isn't a 5 game series. If you think that OU only has a 20% chance to win this game, then that 20% win probability only has to occur on the first game.