Hartline received the promotion after former offensive coordinator Chip Kelly was hired by the Las Vegas Raiders recently.
Ohio State is coming off of a National Title victory over Notre Dame, capping one of the best seasons in program history and taking home the first championship since the new 12-team College Football Playoff format.
Hartline was originally the offensive coordinator in 2023 but head coach Ryan Day handled the play-calling. The former wide receiver has been part of his alma meter's coaching staff since 2017.
Hartline was taken in the fourth round of the 2009 NFL Draft by the Miami Dolphins, where he spent the next six years before heading to the Cleveland Browns in 2015 for his final season.
The former Buckeye had 344 receptions, 4,766 receiving yards, and 14 total touchdowns over the course of his NFL career.