He'll be the fourth starting quarterback for the Sooners in the past four seasons, following Baker Mayfield, Kyler Murray and Jalen Hurts, all of whom were transfers who previously started for other schools.
"Spencer has played very well and certainly very deserving of it," Riley said of the quarterback competition with redshirt sophomore Tanner Mordecai. "I just felt like there was enough separation right now."
Riley said he offered Rattler a scholarship during his freshman year in high school, earlier than he normally likes to. Rattler was ESPN's No. 1 dual-threat quarterback in the country and the No. 29 overall prospect in the 2019 ESPN 300. At Pinnacle High School in Phoenix, he set an Arizona state record with 11,083 passing yards and he threw 116 TD passes. He ran for more than 1,000 yards with 14 rushing TDs in his high school career.
Rattler appeared in three games for Oklahoma last year, going 7-for-11 for 81 yards and a touchdown, with three carries for 23 yards.
Mordecai suffered an injury in fall practice and missed some time while fighting for the starting spot.
"He ended up missing about 70 to 75% of fall camp, which obviously severely limited his true opportunities to really go play his best ball and have a shot at it. But that's football," Riley said.
Mayfield and Murray each won a Heisman Trophy and were No. 1 overall picks in the NFL draft. Last year, Hurts finished as the Heisman runner-up and was a second-round pick of the Philadelphia Eagles in the 2020 draft.
Riley said coaching a young player like Rattler who has never been a college starter, won't affect how he approaches the position.
"I always coach differently," Riley said, noting that he is pleased with depth at the quarterback with Mordecai, freshman Chandler Morris and senior Tanner Schafer. "Guys learn differently, guys need different things and have different strengths and weaknesses. These guys are more than capable of running this offense and playing the position the way we expect it to be played. I think we've got some good players around them, and our expectations for how they play and for how our offense plays, that's certainly not changed."
Riley said it's hard to pinpoint a comparison for Rattler to Murray, Mayfield or Hurts.
"There are some bits and pieces of all of them," Riley said. "He's definitely different than Jalen. Probably a little bit more outgoing, maybe somewhere in between Kyler and Baker a little bit. But he has a natural confidence and kind of charisma about himself. He seems to always have a lot of confidence regardless of the situation that he's in, which I think is key for anybody at that position."