Points Against Ohio State Under Tressel:
2001: 244, 12-games, 20.3/ppga avg., 7-5
2002: 183, 14-games, 13.1/ppga avg., 14-0
2003: 229, 13-games, 17.6/ppga avg., 11-2
2004: 219, 12-games, 18.3/ppga avg., 8-4
2005: 183, 12-games, 15.3/ppga avg., 10-2
2001 Notes [DC Mark Dantonio]: Seven teams scored 20-points or more, OSU was 3-4 in those games. Gave up 34-points to eventual Big-10 champ Illinois in a come-from-ahead-to-lose embarassment of a 4th quarter.
2002 Notes [DC Mark Dantonio]: Only two teams broke 20-points, Texas Tech in garbage time with 2 TDs against the #3s, and Miami, Fla in an overtime game that properly should've ended in regulation with the score 17-14. Six opponents were held to single digits, 5 a single TD or less. The worst game for the defense was in a win where the Silver Bullets surrendered 19-points to Cincinnati.
2003 Notes [DC Mark Dantonio]: Six teams held to 2 TDs or less, including a shutout against Northwestern. Worst defensive game, statistically, was 38-points allowed against Philip Rivers' North Carolina State in an OT game. Worst defensive game, actual performance, was 35-points allowed against Michigan in a game where the Bucks couldn't stop the run.
2004 Notes [DC Mark Snyder]: Five teams held to 2 TDs or less, although six teams broke 20-points. The rebuilt defense was exploited early in the season by mobile QBs, undersized RBs, and bubble screens. Allowed 33-points in road game losses to Northwestern and Iowa.
2005 Notes [DC Jim Heacock]: Four teams held to single digits, seven teams held to 20-points or fewer. Worst defensive performance was against Minnesota where Brian Cupito picked on injured CB Ashton Youboty (hip-pointer) repeatedly en route to 31-points.
Statistically speaking, 2001 was the worst defensive team, and it probably bears out in watching the games. It should be noted though that both Mike Nugent and Josh Huston were freshmen in 2001 and couldn't hit the broadside of a barn. Every PAT was an adventure. Missed kicks took points off the board for the O and gave opponents starting field position at the 35 or better routinely.
EDIT: For the Texas guests, I cannot stress how bad OSU's special teams were in 2001 -- I'm not talking about sliding 45-yarders a foot to the right, I'm talking about doinking 15 yard chip shots off the crossbar. It certainly was not a portent of future special teams success under Jim Tressel, particularly that Nugent would ultimately be a Buckeye hero, All-American, Groza winner, and team captain. The coverages, returns, punts ... all of it stunk. Bad special teams play in 2001 directly resulted in 3 losses, that 7-5 record could have just as easily been 10-2. The UCLA, Wisconsin, and Penn St games all could have, or would have gone differently with a few kicks. The Outback bowl loss to South Carolina came when SC's kicker muscled a 42-yarder through a block and just over the crossbar. Dustin Fox insisted he got a piece of that one.
Dantonio's squads were solid all the way around in 2002 & 2003.
The 2004 defense easily was of a caliber to be under 200-points for the season, but the offense was comically bad and put the defense in a number of bad spots, usually following a string of 3-and-outs in games where the defense could play 35 - 40 minutes. This was Mark Snyder's first and only season as D Coordinator; the common complaint was that he (and his defenses) lacked the intensity and aggression that Dantonio's two previous squads had. The truth was that neither the O or D lines were very good when the season started.
I think the best comparision for the 2006 Bucks is 2004. A ton of talent that has significant real game experience over the prior seasons, they just don't happen to be starters. However, the 2006 D-Line will be far superior, particularly at DT, than the 2004 squad first was, and the D Coordinator is in his second year. Of course, this years' D won't be hamstrung by its offense either.
I don't see any reason why the 2006 Silver Bullets can't be a sub 200-point/16(ish) ppga avg team.