In a stunning move on Sunday, Riley, 38, accepted the head-coaching position at USC.
"Leaving OU was probably the most difficult decision of my life," Riley said in a statement released by Oklahoma. "OU is one of the best college football programs in the country, and it has been forever. ... This was a personal decision solely based on my willingness to go take on a new challenge, and I felt like it was the right opportunity for me and my family to do that.