Gray, who led Tennessee with 772 rushing yards and finished second on the team with 30 receptions in 2020, entered the NCAA's transfer portal last week after the Vols fired head coach Jeremy Pruitt.
The 5-foot-10, 205-pound Gray reportedly was deciding between Oklahoma and Texas, and he posted his decision on Twitter on Wednesday evening.
Gray emerged late in the 2019 season, leading Tennessee with four rushing touchdowns and finishing second with 539 rushing yards on 101 carries. He earned MVP honors in Tennessee's Gator Bowl win over Indiana.
The Memphis native finished his Vols career with 1,311 all-purpose yards and 11 touchdowns (eight rushing, three receiving). ESPN rated Gray as the No. 19 running back and No. 263 overall player in the 2019 recruiting class.
Gray was held out of Tennessee's season finale against Dec. 19 for undisclosed reasons, but sources told ESPN he missed the game because of an internal investigation into potential NCAA violations. In announcing Pruitt's firing last week, Tennessee chancellor Donde Plowman said the internal investigation indicated serious NCAA violations had been committed.
Oklahoma also loses leading rusher Rhamondre Stevenson, who will enter the 2021 NFL draft, while No. 2 rusher T.J. Pledger transferred to Utah. The Sooners averaged 179.7 rushing yards per game in 2020.
ESPN's Mark Schlabach contributed to this report.