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.
''I am thankful for everyone assoc with TOSU for 3 great years and my family and I are excited to join the Houston Texans as the LB coach,'' he posted early Thursday night.
O'Brien, the head coach at Penn State 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.
Ohio State athletic director Gene Smith also turned to Twitter to congratulate Vrabel and his wife.
''Thanks Mike and Jen Vrabel for all you gave the Buckeyes. Good luck in Houston. Here for you always!!! (hash)GoBucks,'' Smith posted on his account.
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.
Follow Rusty Miller on Twitter: http://www.twitter.com/RustyMillerAP
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