His strengths in attack include a good kicking game, footwork, and passing, while in defence despite his lack of size he has a physical approach.
[1] With the Irish sevens team he generally played at centre, but also filled in at fly-half.
[2] O'Brien played for Ireland on the London and Paris legs of the 2017-18 World Rugby Sevens Series; in those tournaments, despite its non-core status, Ireland surprised by finishing third and seventh respectively, with O'Brien scoring four tries across the two tournaments.
[5] On 4 November 2022, he was called up as a late bench replacement to the named squad to face South Africa in the first test of the Autumn Series, following an injury to Robbie Henshaw whose starting place was taken by replacement Stuart McCloskey.
[6] Following an injury to McCloskey in the first half, O'Brien made his test debut, playing 53 minutes in an Ireland victory.