I can only think of two players who have transferred into Ohio State (excluding JUCO/Prep school) ... make that three.... over the last... what... 15 to 20 years... An OL from Bama that didn't play... a CB from Florida State that didn't play... and Anthony Schlegal.
The only recent transfers into Ohio State who had any major impact were OL
Justin Boren (Michigan, 2008) and LB
Anthony Schlegel (Air Force, 2003). Boren was an All Big Ten performer in 2010 and Schlegel had 166 tackles, 17.5 TFLs, and 5.5 sacks in two seasons at Ohio State.
As someone else mentioned, LB
John Kerr (2003) transferred from Indiana. Kerr's younger brother,
Josh Kerr (2006), was on OL who transferred from Miami of Florida. John had 40 tackles in two seasons, while Josh played in only seven games in five years at Ohio State.
In 2001, Tressel brought in a couple of transfers, OL
Mike Kne (2001) from Fordham and DB
Chris Conwell (2001) from Youngstown State. Kne made the two-deep and Conwell played on special teams (8 tackles as a senior in 2002).
The CB from Florida State was
Dionte Allen (2010), who like Jaiden Woodbey spurned Ohio State for his childhood favorite Seminoles. Allen played in four games in 2011 (no stats) after seeing limited action at Florida State.
The OL from Alabama was actually DL
Juan Garnier (2005), who also attended Auburn prior to Ohio State. Garnier was mainly used as a blocker on the FG/XP team.
Long snapper
Harrison Till (2005) and place kicker
Jack Willoughby (2015) transferred in from Duke. Willoughby connected on 7 of 11 FG attempts as a fifth-year senior in 2015.
OL
Ivon Blackman (2010) started his college football career at Robert Morris before transferring to Ohio State. Blackman played in 13 games as a senior in 2013.
And DT
Antwuan Jackson signed with Auburn in 2016, was blocked from transferring to Ohio State, went to a JUCO in 2017, and then signed with Ohio State as part of the Buckeyes' class of 2018.
The above list is probably not exhaustive.