Fox Sports' Bruce Feldman confirmed Tramel's report.
Tramel reported Stoops won't leave the program entirely and is expected to take a different role with the school.
Feldman shared a statement from Stoops about his decision:
ESPN.com's Jake Trotter reported Stoops will provide a further explanation during a press conference Wednesday evening.
Pat Forde of Yahoo Sports reported Stoops had tentatively planned to step down on Friday, but the announcement was accelerated.
ESPN.com's Mark Schlabach shared Barry Switzer's reaction to the news:
Stoops has a 190-48 overall record at the school, and the Sooners have reached a bowl game in each of his 18 seasons. His 190 victories are most all time in program history, while his .798 winning percentage his third behind Switzer and Bud Wilkinson.
Under Stoops' watch, the Sooners won a national championship in 2000 and captured 10 conference championships.
Some, however, feel the program has stagnated in recent years.
Between the Texas Longhorns' decline and conference realignment, the Big 12 isn't the same conference it once was. For Stoops' critics, the 37-17 defeat to the Clemson Tigers in the 2016 College Football Playoff semifinal and 45-24 home loss to the Ohio State Buckeyes last September illustrated the gulf between the Sooners and college football's elite.
After nearly two decades of Stoops in charge, perhaps a new head coach could be a refreshing change for Oklahoma.
Still, Riley will unquestionably have big shoes to fill. Feldman reported Oklahoma doesn't see Riley as an interim coach, adding that those at the school have "been grooming him for this opportunity for over a year."
RedditCFB noted the 33-year-old would be the youngest head coach in FBS if he's confirmed as Stoops' successor.
Hiring Riley would ensure the Sooners maintain some continuity, though, since he has been the team's offensive coordinator since 2015. During that time, Oklahoma ranked seventh and first in S&P+ offensive rating, per Football Outsiders. In 2015, Riley won the Broyles Award, which goes to college football's top assistant coach.