The moves come shortly after the Sooners named Porter Moser the new coach, replacing the retired Lon Kruger.
Harmon was Oklahoma's No. 2 scorer this past season with 12.9 points per game. The 6-foot-2 sophomore missed the Sooners' two NCAA tournament games after testing positive for COVID-19. He started 22 of his 31 games as a freshman the previous year and averaged 7.4 points per contest.
Williams joined Oklahoma as a junior college transfer. He started 14 games this past season as a senior and averaged 6.7 points and 2.8 rebounds. He started 10 games as a junior.
Austin Reaves, Oklahoma's all-Big 12 first-team guard and top scorer last season, has announced his intention to enter the NBA draft pool. Brady Manek has entered the transfer portal as a graduate, forward Victor Iwuakor is leaving the Sooners to play for UNLV, guard Trey Phipps is headed to Oral Roberts and backup center Kur Kuath has announced his intentions to go pro.
Oklahoma got some good news on Wednesday when Moser got his first signee: guard Alston Mason of Blue Valley Northwest High School in Overland Park, Kan. Rivals and 247Sports list him as a three-star recruit. As a senior, he averaged 22.4 points, 4.7 assists and 2.0 steals while shooting 44% from 3-point range.
He joins guards Bijan Cortes and CJ Noland in Oklahoma's incoming freshman class.