I don't think Gene Smith has any intention of firing Holtmann this season absent him mooning the crowd. But say it does happen, or more likely, he leaves for another job. My thoughts on possible candidates.
Sean Miller, Xavier: Clearly one of the strongest candidates - he was great in his first stint at Xavier and hardly a bust at Arizona. He's doing just fine in his second stint at Xavier. He had some pretty public NCAA issues that led to his ouster at Arizona, though he received no sanctions. Given he just started at Xavier, seems unlikely he would jump ship right off the bat.
Greg McDermott, Creighton: Had Holtmann passed in 2017, Greg McDermott would likely have been the guy. He's built a great program at Creighton, though that was after flaming out at Iowa State. The knock on him is that he's pretty similar to Holtmann. He has one Sweet Sixteen appearance and one first place tie in the Big East and one conference win in the Missouri Valley. He's probably the most gettable coach from a major conference.
Chris Mack, Popeyes: Mack is from Ohio and was great at Xavier. That led to him getting the job at Louisville, where they ran him off for reasons that are still pretty weird, at least to me. His time at Louisville wasn't great, though he had a mess to clean up in the wake of Pitino and no support from the university. In any event, he's only 52 and seems destined to take another job at some point.
Jeff Boals, Ohio: Former assistant at OSU, coached at Stony Brook and now Ohio. Has been pretty decent - one tourney appearance and a win. Also probably pretty cheap.
Chris Jent, Lakers: Played at OSU, assistant at OSU. Never been a head coach anywhere. He'd be like the no name Juwan Howard hire.
Billy Donovan, Bulls: I don't follow the NBA much, but Donovan has been there a while to little success. He had lots of success at Florida. At some point, he seems bound to go back to school, though to date he hasn't shown much interest in it and is still under contract with Chicago. I'm sure they would kick the tires here.
Jerrod Calhoun, YSU: ELA mentioned him somewhere. He had a great tenure at Div. 2 Fairmont State. He has slowly turned Youngstown State into a legitimate basketball program, which is saying something. Still, that would be a big departure from what OSU normally does in basketball hires.
Anthony Grant, Dayton: Grant has had a pretty uneventful tenure at Alabama and Dayton, with one exception - his Dayton team was pegged to be a #1 seed when the COVID tourney was cancelled, which is a pretty big youch for a program like Dayton. He fits the profile that Gene Smith seems to like - solid guys at midmajor type schools.
Pat Kelsey, College of Charleston: Long time coach at Winthrop who was successful there, and is currently 26-3 in his second year at the College of Charleston. Seems likely to be a hot commodity. He's from Cincinnati so that's worth something.