Mayfield, a transfer from Texas Tech who walked on at Oklahoma, beat out Trevor Knight and Cody Thomas for the right to start against Akron next week.
Knight will be Mayfield's backup, and Thomas will be No. 3 on the depth chart.
"It's been a very tight battle among those three guys," coach Bob Stoops said in a statement. "Baker, Trevor and Cody have all made substantial progress, and I feel we can win with any of them."
Mayfield will lead the Sooners' new up-tempo, spread scheme implemented by first-year offensive coordinator Lincoln Riley.
Coaches have raved about Mayfield's leadership style, as teammates have gravitated toward his confident, outgoing demeanor. Earlier this summer, Stoops said Baker is "an excellent leader, is highly competitive and has a good delivery."
The 6-foot-1, 209-pound Mayfield was the Big 12 offensive freshman of the year with the Red Raiders in 2013, when he passed for 2,315 yards with 12 touchdowns and nine interceptions.
He sat out last season after Texas Tech declined to give him a release to transfer within the Big 12, even though he was never on scholarship at the school.
ESPN Staff Writer Jake Trotter contributed to this report.
Sooners name Mayfield starting QB
ESPN Big 12 reporter Jake Trotter breaks down why Baker Mayfield was named the starting quarterback at Oklahoma.