Something tells me they'd pursue someone with more of a proven track record than Heupel, unless Heupel has a 9-10 win season this year. I think they'd probably pursue Kiffin or Pittman (who is from Oklahoma and is a former OU position coach) before they'd turn to CJH.
In any event, CJH has a pretty hefty buyout at 8 million until Dec. 15 2022, at which it drops to 6 million. Of course, if Riley leaves, OU would undoubtedly receive a buyout from USC, so paying a buyout for someone else may not be a big deal to them.