Vrabel, who played at Ohio State and went on to a 14-year career in the NFL, will coach linebackers, he confirmed in a post on Twitter.
O’Brien, the head coach at Penn State for the past two years, had been a top assistant with the New England Patriots in 2007-08 when Vrabel was a player. Vrabel won three Super Bowls with the Patriots.
Vrabel was one of the top recruiters for coach Urban Meyer. Joey Bosa, a freshman defensive end, was recruited by Vrabel and developed into a starter this past season. Meyer did not have an immediate comment.
Vrabel becomes the second defensive coach to leave the Buckeyes in recent weeks. Everett Withers, co-defensive coordinator and assistant head coach/safeties left to take the head coaching position at James Madison last month.
The Buckeyes went 12-2 this past season, running their school-record winning streak to 24 games in a row before losing their final two games, 34-24 to Michigan State in the Big Ten championship game and 40-34 to Clemson in the Orange Bowl.