He made his debut for Kerry in the 2014 Munster Senior Football Championship semi-final against Clare and scored two points. He lined out in a first Munster Senior Football Championship when they faced Cork, he scored a point in a 0-24 to 0-12 win and a first Munster title. He missed out on Kerry's win over Galway in the All-Ireland quarter-final. He was back in the starting line up for the semi-final with Mayo scoring a point in a 1-16 each draw.  An injury while out racing on his quad caused him to miss the 2014 All-Ireland Senior Football Championship semi-final replay win over Mayo. However, he started the 2014 All-Ireland Senior Football Championship Final at right half forward. 
Retirements brought new opportunities for OBrien in 2019. He scored Kerrys first goal against Dublin in five games during a 2019 National Football League fixture in Tralee. In the 56th minute of the 2019 All-Ireland Senior Football Championship semi-final against Tyrone, O'Brien scored his fifth championship goal after intercepting a pass from Kieran McGeary, distributing the ball and running the length of the field.