From 2015 to 2017, Grinch was the defensive coordinator at Washington State. He was a semifinalist for the 2017 Broyles Award, given to the nation's top assistant coach.
"Alex has an outstanding track record as a defensive coach and a defensive coordinator," Oklahoma coach Lincoln Riley said in a statement. "He's been one of the most sought-after guys in the country here the last couple of years, especially for the work that he did at Washington State. Taking that defense where he did in a short amount of time is one of the best coaching jobs in the country over the last several years.
"I have a lot of mutual contacts and people I trust in the business who have worked with Alex, and they all speak highly of his ability to not only be one of the best schematically, but also a guy who will help build the culture that we want and expect here at Oklahoma on the defensive side."
Grinch joined Ohio State last year and was co-defensive coordinator alongside Greg Schiano. He also worked with the Buckeyes' safeties.
Grinch will be charged with resuscitating an Oklahoma defense that ranked 92nd in defensive efficiency this season. Oklahoma fired defensive coordinator Mike Stoops in October, replacing him with Ruffin McNeill on an interim basis.
"This is a unique opportunity at one of the premier institutions under elite leadership," Grinch said. "Oklahoma is in the top 1 percent. There's no faking it in this business. You're either it or you're not. Oklahoma is one of those programs."
The Sooners fell to Alabama 45-34 in the College Football Playoff semifinal to become the first team in Oklahoma history to allow 45 or more points five times in a season.