OU's issue is the recruiting part. Lincoln Riley was the one bringing in those recruits, as you can see from the mass exodus when he left. He was getting many of them from California, which will likely be done with now, as it looks like all the kids in Cali want to go to USC.
So yes, focusing on the defense will be great, but you have to pull the elite recruits from somewhere. If the Cali pipeline is closed with Riley at USC and the Texas schools now basically putting players on the payroll with NIL deals. It will get tougher and tougher for OU to pull those elite 5 star recruits Riley was pulling in.
If the goal is to compete in the Big 12 -- then yes, I agree, the team will be fine. If it is to compete for national titles -- then I think OU fans aren't going to be happy with the new hires. I don't see any way OU can recruit with the top teams if you have Riley locking down California. With Texas and Texas AM now shelling out serious NIL money -- OU is going to find it much tougher out there recruiting in the upcoming years.