O'Brien made his senior competitive debut for Leinster in their round 8 2019–20 Pro14 interprovincial clash against Ulster on 20 December 2019, with O'Brien starting at 13 for Leinster and playing 70 minutes in their 54–42 win against the northern province. O'Brien is 10 points behind Michael Bent on the all time Leinster points list.