Career
Watford
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. 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.
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.
Wycombe Wanderers
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.