Urban Meyer and Jon Gruden have been rumored to be two of the top candidates for the job if it opens. Meyer is probably the most logical choice, and even though he just retired, Ohio State is probably the one job that would make him come out of retirement. OSU is a top 3 job in the country, and they will give him a ton of money. Gruden is a possibility also. He's said that he wants to get back into coaching, and even though he was a high-level NFL coach, OSU is a pretty premier coaching gig, and he was recently at OSU to speak at a coaches clinic, and has watched OSU practice multiple times.
If OSU doesn't get either of those 2, Bo is probably the next choice. Remember that OSU hired Tressel when he was at Youngstown State, so its not like they are only going to look for guys who have won big already.