USC hopes the hire will result in a return to national prominence for the football program. Riley, 38, is considered to be one of the best coaches in college football.
"This was the right time. Everything about it made sense ... I love the situation we're in right now," Riley said at the press conference.
Riley left Oklahoma for the Trojans. Riley went 55-10 in five seasons leading the Sooners, winning four Big 12 titles and making three College Football Playoff appearances in his first head coaching job.
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