And it's probably something like they make decisions early on how their defense is going to work and it's not working at first and you keep fighting through it and then by the time you realize you need to go a different direction, it's too late.
I'm referring to their plan to switch basically everything, except sometimes they're just switching 1-4. A lot of coaches hate this because it allows defenders to be lazy -- you just stay with your guy until he screens, is screened for, or just crosses paths with other offensive players -- by picking up whoever you come across. It requires players to defend players in their "zone" (so to speak) very closely and also to communicate very clearly with all their switches. OU did this all season and it worked out badly. I'm sure they worked and worked and worked and it never got better and by the time they were ready to give up, it was too late to change the entire defense's philosophy.
The other thing that OU's defense did all year long -- and I don't know if this was taught and preached by the coaches or if it just happened due to a lack of discipline -- is collapse on dribble penetration which left 3 point shooters open on the perimeter for wide open shots. With rim protectors inside, this isn't necessary. OU routinely got killed on open 3 point shots which is what the other team wanted because no one nowadays wants to shoot 12 foot contested shots in the lane. It didn't help that Trae Young was absolutely incapable of stopping or even slowing dribble penetration but it was often compounded by the team's switching because we'd end up with someone like Manek or McGusty on the guy with the ball who would then get into the lane easily, force perimeter defenders who are still trying to match up with the new guy they're defending from all the switching to collapse into the lane, leaving good shooters wide open on the perimeter.
This has to stop next year. Good defensive teams have a rim protector inside -- and OU does -- and doesn't leave 3 point shooters on the perimeter to help. If a team wants to shoot jumpers from the free throw line or have a 6'1" PG take on McNeace inside, by all means, let them. They cannot continue collapsing and leaving wide open shooters. One thing that would help is a PG who can slow dribble penetration. It's nearly impossible to stop completely but we need to have someone who makes it tougher.