I'm not going to be weird and hang out on your board but I'll clear up why Heupel left Oklahoma.
Heupel and Stoops had different visions for what they wanted the offense to evolve into for the future. Coach Stoops wanted to incorporate power run into the spread. Huepel basically said that's not the offense I run or the one you hired me to run. This resulted in Huepel never getting full control of the offense from Stoops and some bad vibes started to form as the offense, while productive, hit a wall.
Then comes coach Stoops shiny new toy Lincoln Riley who ran the offense Stoops wanted for the future.
Heupel was eventually pushed out and its reported he was fired over the phone. Which you can all imagine he didnt appreciate.
And that's it. He immediately went on to coach offense for other schools until the head coaching gigs came along and now he's with you guys.
It's not an interesting story but it's the background you all didnt have on him. He didnt fail at Oklahoma. Oklahoma and Heupel just grew apart.