He inherited a team on a 10-game winning streak that was defending Big XII Champions, that had demolished Auburn in the Sugar Bowl, and had an excellent recruiting class coming in.
He did go to three straight playoff games, but OU got progressively softer each year he was there. In his first season, they were competitive with Georgia, losing 54-48 in double overtime. The next year, they got blown out by Alabama (although they scored some garbage points in the second half to make the final score more respectable). In his final playoff appearance, Lincoln and Alex Grinch got run out of the building by LSU in a 63-28 trouncing, giving up almost 700 total yards to the Tigers. Then, there's this year. OU failed to qualify for the Big XII Championship and were a handful of plays away from finishing 7-5. In the process, they made Tulane look like an SEC team, and got dominated by the worst team in the FBS (Kansas) for three quarters. If I was a USC fan, my biggest concern would be Alex Grinch.
Riley may or may not succeed, but he's built nothing to this point. He took over as head coach at OU standing on 3rd base, and apparently a lot of people think he hit a triple. As to his two Heisman quarterbacks, he deserves some credit, but they were transfers. His hand-picked, highly recruited, can't miss, Heisman anointed QB, Spencer Rattler, actually regressed this year before getting benched.