If Meyer is fired or allowed to resign, the most likely scenario is that OSU sticks with an interim coach this season.
After that, I would fully expect that Chris Peterson and Tom Herman would be the guys they target (I don't think the university will blink at buy-outs, especially if Meyer is fired for cause and his buyout is null and void). Unless he totally bombs this season, Jeff Brohm more than likely will be on the short list as well, especially if Purdue explodes this year. That said, I still believe that Brohm's ultimate goal is the NFL.
I don't see it happening, but Bob Stoops' name will be bounced around. He has the Ohio ties and the pedigree, but also has his own share of baggage.
If Campbell continues to show a trajectory at ISU and wins at least 9 games, he certainly will be on the list, but I'd be surprised if he's higher that #6 or #7 right now.