I was thinking that a chart might help to visualize this so here it is:

Ohio State's and Indiana's lines are contiguous. The other nine lines have a break because you don't play yourself.
Start with looking at Ohio State's line. It starts at the left with Ohio State's worst record against any Big11Ten/B1G team which is .633 on 19-11 against Michigan then generally ascends as you move across reaching a peak of 1.000 on 27-0 against Indiana. There are two oddities that jump out to me:
Purdue:
Ohio State is #1 against nine of the 10 teams listed here and the exception is Purdue. It is a BIG exception. Not only is Ohio State not #1 against the Boilermakers, they aren't even in the top-3. The Buckeyes have the fourth best record against Purdue at .750 on 15-5 which trails PSU (.889), Wisconsin (.827), and Michigan (.789). I find it kinda funny that while Purdue is the one team that Michigan did better against than Ohio State, the Wolverines still aren't #1 because they were outdone by PSU and UW.
Illinois:
The Buckeyes are #1 against Illinois but it is REALLY close. If you look at it, all the records against Illinois are bunched up. I *THINK* this is because if you think about Illinois' history they really aren't a consistently moderately bad team. Instead they have tended to be downright awful most of the time and then a title contender once in a while. Consequently, they are better than their peers at beating tOSU and Michigan because when they are good, they tend to be REALLY good but then the rest of the time they just suck so they aren't as good as you might expect at beating Indiana, Minnesota, and especially Purdue. Illinois has the smallest top-to-bottom gap in winning percentage against:
- .818 Ohio State, 18-4
- .813 Michigan State, 13-3
- .796 Wisconsin, 21-5
- .789 Michigan, 15-4
- .783 Penn State, 18-5
- .720 Purdue, 18-7
- .708 Iowa, 17-7
- .613 Northwestern, 19-12
- .577 Minnesota, 15-11
- .455 Indiana, 10-12
The only top-to-bottom gap in winning percentage against that is remotely close to that small is for Ohio State where the best is Michigan's .367 and the worst is Indiana's 0.000.