O'Neil said that the player has already got plenty of talent but with his age can only see improvement from him over the coming years.
"I'm looking forward to working with him, I really am," O'Neil told Cleveland.com. "He flashes some on tape.
"He did a lot of good things. He's obviously a young player, he's got some things to work on but we're excited as a defensive staff to get him back here and start working with him."
Mingo started in three of Cleveland's regular season games in 2013.