Luke Terry O'Nien (born 21 November 1994) is an English professional footballer who plays for Sunderland, as a midfielder. O'Nien joined Watford as an Under-9 and impressed throughout his time in the youth ranks with his technical influence on the pitch. Having put pen to paper on his first professional contract at Vicarage Road in July 2013, O'Nien 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 number thirty-two shirt. O'Nien had been an unused substitute on a handful of occasions during the 2013–14 season. The midfielder finally 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. O'Nien 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. O'Nien 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. O'Nien made thirty-five appearances for them and scored five times – against Weston-super-Mare and Basingstoke Town. His second loan spell at Wealdstone proved to be successful for O'Nien. 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.