Gary O'Neill became Norwich's third signing of the 2014 pre-season when he signed a two year deal on 5th August 2014.

O'Neill began his career at Portsmouth, progressing from their youth team and making his debut in January 2000 against Barnsley as a sixteen year old. After loan spells at Walsall (2003) and Cardiff (2004) he went on to make 174 appearances, scoring 16 goals before joining Middlesbrough for approximately £5million on 31st August 2007. He made 109 appearances for Boro, scoring eight goals, before returning south to join West Ham in January 2011. He played 48 games for the Hammers scoring three goals

on 7th August 2013, he joined QPR on a one year deal. He went on to make 29 appearances as he helped them to promotion from the Championship.

O'Neill made his Norwich debut as a substitute in the 3-0 win over Watford on 16th August 2014.

He made 24 appearances in 2014/15 as City won promotion back to the Premier League, including coming on as substitute in the 2-0 Championship Play-off final victory over Middlesbrough on 25th May 2015

He made 29 appearances for City in 2015/16 as the Canaries were relegated from the Premier League.

Despite offer of a new contract with Norwich, O'Neill opted to join Bristol City signing a two year contract witj Bristol City on 9th June 2016. He was released at the end of the 2017/18 season having made 36 appearances for the Robins, scoring once.

On 3rd August 2018, O'Neill signed for Bolton on a short term deal until January 2019. This was extended on 18th January until the end of the season. On 19th April 2019, Bolton were relegated from the Championship following a 2-0 defeat to Aston Villa. It was their 14th defeat in 18 games.

O'Neill left Bolton on 1st July 2019. He joined Aberdeen on 25th October 2019 but didn't play during the 2019/20 season due to an Achilles injury which led to his retirement.

On 10th August 2020 O'Neill was appointed as Liverpool's Assistant Manager for their Under-23 squad. On 25th February 2021 he was appointed as first team coach at Bournemouth along with Joe Jordan, working under new manager Jonathan Woodgate.

Gary has been capped at Engand Under-19, Under-20 and Under-21 levels.