He's only 56 yrs old... he's enjoyed a remarkable career at OK, 190 wins versus only 48 losses. There would be numerous opportunities available to him in the NFL, me thinks.
Maybe he decided that he's served his time and he's ready to call it a career... but I would be shocked if a year from now he's not taking a job somewhere else, perhaps in the NFL.
Then again, his father passed away at the age of 52, and maybe he's looking at things from a completely different perspective. Either way, I wish him the best. He's always seemed to be a genuinely good guy who ran a very respectable program.