John Cooper went 46-3 against unranked teams from October 17, 1992 through the end of 1998. The 3 losses were pretty costly
Quite possibly cost Ohio State two NC's.
Loss to unranked Michigan in Ann Arbor on 11/20/1993:
Ohio State came into this game 9-0-1 (tied UW in Madison) and #5 in the AP Poll. That may not sound all that close to the top but based on what happened after that, it was. On the same day that Ohio State lost in Ann Arbor, #1 Notre Dame (just moved up fresh off their GOTC win over previous #1 FSU in the first-ever on-campus Gameday game) lost at home to Boston College. Also, #4 Miami lost in Morgantown to #9 WVU.
#2 in the poll was 9-1 FSU and the Seminoles ended up winning the NC.
#3 in that poll was 10-0 Nebraska, they lost to FSU in the Orange Bowl.
Wisconsin had previously and inexplicably lost in Minneapolis so the Buckeyes were alone atop the Big11Ten standings and a win in Ann Arbor would have sent them to Pasadena. A win there over UCLA (whom UW did beat there) would have left Ohio State as the lone (non-probationary) unbeaten and earned them the NC over 12-1 FSU (lost to ND), 11-1 ND (lost to BC), and 11-1 UNL (lost to FSU).
Loss to unranked Illinois in Columbus on 10/8/1994:
Ultimately this one didn't cost much as the Buckeyes finished the regular season 9-3. An improvement to 10-2 wouldn't have made much if any difference in bowl destination and the NC was obviously off the table based on the blowout loss in Happy Valley.
Loss to unranked Michigan State in Columbus on 11/7/1998:
I still don't know how this happened. I missed the ending due to my presence being demanded at a family wedding (inconsiderate to get married on a fall Saturday). Ohio State was #1 going into this one and ended up just BARELY missing the inaugural BCSCG. The Buckeyes were #2 in the final poll after beating aTm in the Sugar Bowl.