The 6-foot-2, 244-pound Lewis was a second-team All-Big 12 selection by The Associated Press and is rated as the No. 4 outside linebacker by ESPN's Scouts Inc.
"Ronnell leaves with our best wishes," said Oklahoma coach Bob Stoops in a statement Thursday. "He managed several challenges while playing here, but always worked hard and gave us his very best effort. He has the skill to be a successful NFL player and we feel he'll do well there."
Lewis, a former 8-man football star at Dewar (Okla.) High School, flourished this season, his first as a full-time starter. He had 5.5 sacks and 13 tackles for loss.
He had previously been on ESPN draft expert Mel Kiper Jr.'s Big Board until a late-November knee injury forced him out of the lineup. Lewis was suspended for the season finale against Oklahoma State because he had skipped too many classes, and he also missed their 31-14 win over Iowa in the Insight Bowl on Dec. 30 because of academic issues.
Quarterback Landry Jones announced Thursday he is staying at Oklahoma for his senior year.