In the actual NCAA report it states that both the NCAA and OU were sent an anonymous email. The report can be found here just search for Oklahoma.
https://goomer.ncaa.org/wdbctx/LSDBi/LSDBI.home
"In its response to the enforcement staff's notice of allegations, the institution claimed that it "self-reported" the employment violations to the NCAA. In reality, the violations at the dealership were first exposed in a March 3, 2006, electronic message (e-mail) sent by an anonymous source to both the NCAA enforcement staff and to institution officials. This e-mail stated that several student-athletes, including student-athlete 1, the institution's starting quarterback, were paid by the dealership for work that they did not perform. But for this e-mail, it is unlikely that these violations would have come to light"