1. Gareth O'Brien previously played for the Warrington Wolves and the Salford Red Devils in the Super League, and on loan from Warrington at the Widnes Vikings, Castleford Tigers and St Helens in the Super League, the Swinton Lions in the Championship and the North Wales Crusaders in League 1.
Gareth O'Brien later joined the Tigers on a permanent deal.
Gareth O'Brien is a product of the Warrington academy; he was seen as the long-term replacement for Lee Briers.
Gareth O'Brien made his Super League debut in Round 14 against Castleford, partnering Lee Briers as the half-backs.
Gareth O'Brien featured in Round 4 and 5 and the quarter-final of the Challenge Cup against Keighley, Bradford and Catalans.
Gareth O'Brien scored against Salford, Widnes, Hull KR, Leeds, Hull FC, Catalans and Bradford.
Gareth O'Brien featured in Round 5 of the Challenge Cup against Keighley.
Gareth O'Brien scored against Salford, Widnes and Keighley.
Gareth O'Brien played in Round 5 of the Challenge Cup.
Gareth O'Brien featured in the playoffs in the Elimination Play Off to the Qualifying Semi-final.
Gareth O'Brien scored against Widnes, Bradford Bulls, Castleford, Huddersfield and St Helens.
Gareth O'Brien played in Round 17 to Round 18 then in Round 20 to Round 23.
Gareth O'Brien featured in the Challenge Cup in Round 6 to the semi-final.
Gareth O'Brien scored against Hull FC, Catalans Dragons, St Helens, Huddersfield, Widnes, Castleford, Leigh Centurions, Wigan Warriors, Wakefield Trinity Wildcats and Hull Kingston Rovers.
Gareth O'Brien was released by Warrington at the end of the 2015 season, alongside teammates Joel Monaghan and Roy Asotasi, as the club continue their on and off-field changes.
In March 2012, Gareth O'Brien was sent out on loan for a month to Super League side Widnes Vikings.
In 2013 and 2014, Gareth O'Brien played for the Swinton Lions in the Championship through their dual registration arrangement with Warrington.
Gareth O'Brien made 7 appearances and scored 6 goals for the Lions in 2013, and made a further 8 appearances and scored 2 tries, 34 goals and 2 drop goals for the club in 2014.
In February 2013, Gareth O'Brien joined the Castleford Tigers on a one-month loan deal.
Gareth O'Brien was signed as cover for Tigers half-back Rangi Chase, who was serving a suspension.
Gareth O'Brien made his debut in Round 3 against the Bradford Bulls, and then featured in Round 4 against the Catalans Dragons when his final-moment drop goal earned a draw for Castleford.
In May 2013, Gareth O'Brien joined St Helens on a one-month loan deal.
Gareth O'Brien then kicked 8 goals against the Salford City Reds, and 1 goal against Hull Kingston Rovers.
In May 2015, Gareth O'Brien played for the North Wales Crusaders in League 1 through their dual registration arrangement with Warrington.
Gareth O'Brien joined the Salford Red Devils for the 2016 season and was assigned squad number 14.
On 12 March 2018, having made 4 appearances and scored 1 try and 9 goals in the 2018 season, it was announced that Gareth O'Brien would leave Salford to join Toronto Wolfpack with immediate effect.
Gareth O'Brien joined Toronto Wolfpack in the Championship for an undisclosed fee on 12 March 2018.
Gareth O'Brien said he "was really impressed" by the club and that joining the Wolfpack was an opportunity he couldn't afford to turn down.
Gareth O'Brien had made 5 appearances and scored 1 try and 7 goals for Toronto when the 2020 season was suspended due to the COVID-19 pandemic.
Gareth O'Brien arrived as a direct replacement for full-back Jordan Rankin who had been released during the season suspension to allow him to return to Australia.
Gareth O'Brien made 8 appearances throughout the remainder of the season, playing at fullback and scrum-half.
Gareth O'Brien scored 3 tries, 7 goals and 1 drop goal in this time.
In Castleford's round 3 Challenge Cup match against Hull KR, Gareth O'Brien kicked a 99th-minute drop goal to secure a victory for the Tigers in the fourth period of golden point extra time.
On 20 October 2022, Gareth O'Brien signed a contract to join the newly promoted and rebranded Leigh side.
Gareth O'Brien played 27 games for Leigh in the 2023 Super League season as the club finished fifth on the table and qualified for the playoffs.
Gareth O'Brien played in their elimination playoff loss against Hull Kington Rovers.
Gareth O'Brien played 19 games for Leigh in the 2024 Super League season which saw the club finish fifth on the table.