Let's look back at the last decade, and the years with the best defenses. Let's list how many yards/game they gave up, and points/game. Ranking them in by points given up:
2006 - 303, 18.1 9-4/MAC East/Bowl
2009 - 348, 21.3 9-4/MAC East/Bowl
2011 - 361, 22.1 9-4/MAC East/Bowl
2010 - 353, 23.8 8-4/Bowl
2012 - 389, 24.8 8-4/Bowl
2014 - 394, 24.8 6-6
2013 - 406, 27.5 7-5/Bowl
2008 - 349, 28.2 4-8
2007 - 408, 29.9 6-6
2005 - 453, 30.5 4-7
The correlation is pretty obvious.
How good can this defense be? I'm thinking it will be comparable to the 2009-2011 defenses, but they seem to have a nasty streak that reminds me of the 2006 defense, so I'm hoping they will surprise me and hold opposing teams to under 21 points a game.
Let's also do the same thing for offense:
2011 - 446, 33.5 9-4/MAC East/Bowl
2012 - 445, 31.7 8-4/Bowl
2007 - 363, 30.5 6-6
2010 - 326, 27.5 8-4/Bowl
2013 - 379, 27.4 7-5/Bowl
2009 - 310, 24.8 9-4/MAC East/Bowl
2008 - 361, 24.1 4-8
2014 - 370, 20.5 6-6
2006 - 277, 19.7 9-4/MAC East/Bowl
2005 - 293, 17.5 4-7
There isn't a lot of correlation here. On the other hand, obviously since the defenses were the same between 2012 and 2014, the better offense in 2012 made a big difference. Similarly, the very good offense in 2007 kept that team from being as bad as you would have expected from the defense.
How good will the 2015 offense be? I think this offense will be effective particularly at moving the ball on the ground. That doesn't lend itself to huge offensive numbers like 2011 and 2012. Instead, I see this offense as similar to 2008-2010. Therefore I'm thinking in terms of an offense that averages 24 to 27 points a game.
Suppose the offense does average 26 a game, and the defense does average 21 a game. That's a +5 margin. Let's look at the data one last time, this time using the average margin:
2011: +85,+8.4 9-4/MAC East/Bowl
2012: +56,+6.9 8-4/Bowl
2010: -27,+3.7 8-4/Bowl
2009: -38,+3.5 9-4/MAC East/Bowl
2006: -26,+1.6 9-4/MAC East/Bowl
2007: -45,+0.6 6-6
2013: -26,-0.1 7-5/Bowl
2008: +13,-3.1 4-8
2014: -24,-4.1 6-6
2005: -160,-13.0 4-7
There is obviously strong correlation here, too, but one other thing stands out. I notice that only 3 of the ten teams averaged more yards than they gave up, while 6 of ten averaged more points than they gave up.
Anyway, if I am right, and Ohio does average +5 more points than they gave up, this should be a pretty good team.