The South Korean found it difficult to break into the first-team last year and had limited playing time. However, Ji claimed a bronze medal in the Olympic Games with his country, and his club boss is keen for him to help the accolade inspire him to success with the Black Cats.
"I'm hoping that the Olympic experience has done Ji the world of good," O'Neill told the club's official site. "I'm sure it has.
"South Korea were quite successful and they're expecting to reach anything up to the quarter-finals [in any tournament], so good have they become of late.
"I hope he comes back refreshed and ready to go; we've given him a little bit of time off after the championships but he'll be back towards the end of the month to stake his claim for a place. It's up to him, it really is. It's a fight now and I'm hoping he's up for the battle."
Ji's finest moment last season came when he scored an injury-time winner at home to Manchester City on New Year's Day.