His father Garrett was captain of the Offaly team which won the 1988 All-Ireland Under-21 Football Championship, the only such victory in the county's history.
[citation needed] Conor took up rugby as an 11 year with Mullingar RFC, he tried out many positions including back-row before finally settling at out-half.
He was the out-half on the Leinster U18 team that won the interprovincial title in 2012/13 which lead to his selection for Ireland U18 in four matches against Italy, England, Scotland and France.
[citation needed] O'Brien joined the Leinster Rugby academy ahead of the 2016–17 season, during which he also won an All-Ireland League title with Clontarf.
O'Brien then starred as Ireland finished in their best ever position of second at the 2016 World Rugby Under 20 Championship, during which they also beat the Baby Blacks for the first time.