O'Nien joined Watford as an Under-9. Having signed his first professional contract at Vicarage Road in July 2013, he was then called up by the first team to the pre-season friendly tour in Italy after he impressed staff during training. O'Nien was then given the number 32 shirt. He had been an unused substitute on a handful of occasions during the 2013–14 season. He made his Watford debut as an 88th-minute substitute in a 3–0 win at home to Barnsley on 15 March 2014. After the match, O'Nien reacted to his debut against Barnsley. On 27 March 2014, O'Nien signed a one-month loan deal with Isthmian League Premier Division side Wealdstone. He made his debut as a late substitute in a win over Billericay Town on 31 March 2014, while his first start came three days later on 3 April 2014 in a 3–0 success over East Thurrock United. He then started the final game of the season, a 2–0 home win against Canvey Island. Following his loan spell, on 13 May 2014, O'Nien signed a new one-year deal with the Hornets. He expressed delight over signing a new contract with the club. In August 2014, O'Nien returned to Wealdstone, now a Conference South side, on a season-long loan deal. He made 35 appearances for them and scored five times – against Weston-super-Mare and Basingstoke Town. His second loan spell at Wealdstone proved to be successful. Following Watford's promotion to the Premier League in 2015, O'Nien was one of three released by the club following the expiration of his contract.
On 25 July 2015, O'Nien signed for League Two side Wycombe Wanderers on a one-year deal, having had a successful trial with the club. In 2017, he earned the accolade of Community Player of the Season. In the 2017–18 season, the club won promotion to League One.
Sunderland
O'Nien joined Sunderland on 30 July 2018. He scored his first goal for the club in the 2–0 away game win against Shrewsbury Town in October 2018. His second goal for the club came on 27 November 2018 and it was Sunderland's 4th, and final, goal of an entertaining 4–2 home victory against promotion rivals Barnsley. 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.
