Selected in the Ireland under-20s for the 2018 Six Nations Under 20s Championship, O'Sullivan started against France, Italy, Wales, Scotland, and England, scoring four tries and earning two Man-of-the-Match awards during the tournament. O'Sullivan again won selection for the under-20s, this time for the 2018 World Rugby Under 20 Championship, but he suffered a knee injury just days after being selected, which ruled him out of the tournament.