The 6-foot-5 guard Grandstaff, a top-50 player coming out of high school in Texas, also considered SMU.
"They have really turned the program around, and they do everything the right way," Grandstaff told ESPN of why he chose the Sooners. "The players work really hard and are really close. They let their guards play and those guys have all really improved since they got there."
Grandstaff was considered one of the top perimeter shooters in the freshman class but was receiving limited playing time early in the season with the Buckeyes. He averaged 4.4 points in 10 games this season before deciding to transfer.
Grandstaff will be eligible after the first semester next season.