In March 2019, after consistent performances in an unfamiliar right back role in which he won multiple man of the match awards. At the end of the 2018–19 Season O'Nien picked up the award of Young Player of The Season. In his first season at Sunderland, O'Nien got attacked by a Portsmouth fan at Fratton Park after he got accidentally pushed into the stands by a player on the opposing team. O'Nien appeared to get punched, kicked and hurled abuse at by nearby Portsmouth fans. However, O'Nien wasn't injured and didn't end up pressing charges. O'Nien was born in Hemel Hempstead, Hertfordshire. His father Terry was a professional golfer. O'Nien qualifies to play international football for both England and Singapore. He qualifies to play for Singapore through his Singapore-born maternal grandfather; his great-uncle was politician Lim Kim San. Sunderland