O'Brien was born in Warrington, Cheshire, England. Gareth is a product of the Warrington academy, he was seen as the long term replacement for Lee Briers. Gareth is the ambassador for the Rett syndrome research trust , even posing as Mr. July in their Natural Wire Calendar, in which several players posed nude.
Warrington
Statistics do NOT include pre-season friendlies. 2011 - 2011 Season O'Brien made his début in the 80-0 Challenge Cup win over Keighley, he marked his début with a try. He made his Super League début in Round 14 against Castleford, partnering Lee Briers as the half-backs. O'Brien steered the team to a 62-0 win crossing the line for a try himself. Gareth then featured in the record breaking 112-0 home victory over the Swinton Lions in the 4th Round of the Challenge Cup. An injury prevented him from playing for the rest of the season. 2012 - 2012 Season O'Brien featured in Round 10 to Round 20. He also featured in Round 4 and 5 in the Challenge Cup against Keighley and Bradford Bulls. He scored against Salford, Widnes, Hull Kingston Rovers, Leeds, Hull F.C., Catalans Dragons and Bradford Bulls. 2013 - 2013 Season O'Brien featured in Round 7 and Round 9. O'Brien featured in the Challenge Cup against Keighley. He scored against Salford, Widnes and Keighley. 2014 O'Brien featured in Round 1 to Round 6. Gareth then featured in Round 9 to Round 11. Gareth then played in Round 18 to Round 27. O'Brien played in Round 5 of the Challenge Cup. Gareth also featured in the playoffs in the Elimination Play Off to the Preliminary Semi Final. He scored against Widnes, Bradford Bulls, Castleford, Huddersfield and St. Helens. 2015 Gareth featured in the World Club Series against St George Illawarra Dragons. He scored against the Dragons. O'Brien featured in Round 1 to Round 9. He played in Round 17 to Round 18 then in Round 20 to Round 23. O'Brien also featured in the Challenge Cup in Round 6 to the Semi Final. He scored against Hull F.C., Catalans Dragons, St. Helens, Huddersfield, Widnes, Castleford, Leigh Centurions, Wigan Warriors, Wakefield Trinity Wildcats and Hull Kingston Rovers.
Widnes
2012 - 2012 Season O'Brien was sent out on loan for a month to Super League side Widnes. Gareth made his début in Round 6 against Wigan where he kicked 6 goals to help the Chemics to a 37-36 victory. He played in Round 7 against Hull F.C. but could only manage a goal as Widnes slipped to a 58-10 defeat. He featured in Round 8 and kicked 7 goals to help Widnes to a 38-30 win. Gareth converted Widnes' only try in the 76-6 loss against Catalans Dragons in Round 9.
St Helens
O'Brien was sent on loan to St. Helens for a month in 2013. He made his début against Huddersfield, and kicked 2 goals in the 25-16 loss. He then featured against Bradford Bulls. O'Brien then kicked 8 goals against the Salford iry Reds, and 1 goal against the Hull Kingston Rovers. Gareth then kicked 6 goals in the 40-24 victory over the Castleford Tigers. O'Brien helped the Saints to a famous 22-16 victory over arch rivals, the Wigan Warriors by kicking 2 goals. Gareth also kicked a goal in the 26-6 victory over the Catalans Dragons. In 2016 he was called up to the Ireland squad for the 2017 Rugby League World Cup European Pool B qualifiers.
On 11 March 2018, he signed for Canadian side Toronto Wolfpack due to Salford Red Devils been in poor financial state after the departure of Chairman; Dr Marwan Koukash.
