O'Neil signed for Middlesbrough on 31 August 2007 for an undisclosed fee, believed to be in the region of £5 million.
O'Neil, whilst not scoring for Middlesbrough during his first season, was considered a success.
His lively displays down the right-hand side of midfield, and occasionally in the centre, have earned him plaudits from the Boro fans and manager, Gareth Southgate. "
I think he's done a fantastic job on the right hand side this year, I think there's more to come from him".
In all he made 26 appearances in the league, and three in the FA Cup during the club's run to the quarter-final.

Reports surfacing in May 2008 suggested that O'Neil might look to move on from Middlesbrough after he and his family had failed to settle on Teesside.

O'Neil scored his first Middlesbrough goal in the 2008–09 season on 29 October in Middlesbrough's 2–0 win over Manchester City.

O'Neil started the 2009–10 season as Southgate's first choice central midfielder, with Julio Arca being dropped to the bench for the first matches of the season.
It was soon revealed that O'Neil would require a hernia operation, yet he spoke out and said he would, "Delay the operation so as to ease Boro's midfield crisis."
The first game after his operation, on 12 September, led to his first goal.
He was taken down in the box yet managed to pull himself up to loop Marvin Emnes' return ball over the Ipswich Town keeper.
The game ended 3–1 in Middlesbrough's favour.
Just three days later, on 15 September, he was taken down with a head injury bleeding against Sheffield Wednesday and required stitches, so was taken off the pitch for treatment and returned minutes later.
The game ended 3–1, with O'Neil playing an important part in Boro's midfield dominance.
O'Neil managed 119 appearances and scored seven goals in total for Boro.
O'Neil signed for Middlesbrough on 31 August 2007 for an undisclosed fee, believed to be in the region of £5 million. O'Neil, whilst not scoring for Middlesbrough during his first season, was considered a success. His lively displays down the right-hand side of midfield, and occasionally in the centre, have earned him plaudits from the Boro fans and manager, Gareth Southgate. "I think he's done a fantastic job on the right hand side this year, I think there's more to come from him". In all he made 26 appearances in the league, and three in the FA Cup during the club's run to the quarter-final.
Reports surfacing in May 2008 suggested that O'Neil might look to move on from Middlesbrough after he and his family had failed to settle on Teesside.
O'Neil scored his first Middlesbrough goal in the 2008–09 season on 29 October in Middlesbrough's 2–0 win over Manchester City.
O'Neil started the 2009–10 season as Southgate's first choice central midfielder, with Julio Arca being dropped to the bench for the first matches of the season. It was soon revealed that O'Neil would require a hernia operation, yet he spoke out and said he would, "Delay the operation so as to ease Boro's midfield crisis." The first game after his operation, on 12 September, led to his first goal. He was taken down in the box yet managed to pull himself up to loop Marvin Emnes' return ball over the Ipswich Town keeper. The game ended 3–1 in Middlesbrough's favour. Just three days later, on 15 September, he was taken down with a head injury bleeding against Sheffield Wednesday and required stitches, so was taken off the pitch for treatment and returned minutes later. The game ended 3–1, with O'Neil playing an important part in Boro's midfield dominance. O'Neil managed 119 appearances and scored seven goals in total for Boro.
Gary O'Neil