[2] After returning from a long-term knee injury sustained whilst on international duty with Ireland under-20s, O'Sullivan was selected in the Munster A squad for the Cara Cup, a tournament hosted in Weymouth, Massachusetts,[8] starting in the 38–19 win against New England Free Jacks on 6 April 2019.
[9] O'Sullivan made his senior competitive debut for Munster on 28 September 2019, featuring as a replacement in the provinces opening 2019–20 Pro14 fixture against Welsh side Dragons in Thomond Park,[10] and he made his first start for the province in their 28–12 win against Welsh side Ospreys on 25 October 2019.
[11] O'Sullivan made his European debut for Munster in their final pool 4 fixture of the 2019–20 Champions Cup against Welsh side Ospreys on 19 January 2020.
[13] In Munster's 29–10 win against Welsh side Scarlets during round 13 of the 2019–20 Pro14 on 29 February 2020, O'Sullivan scored his first try for the province,[14] and he won the John McCarthy Award for Academy Player of the Year for his performances during the 2019–20 season.
[16] Selected in the Ireland under-20s for the 2018 Six Nations Under 20s Championship,[17] O'Sullivan started against France,[18] Italy,[19] Wales,[20] Scotland,[21] and England,[22] scoring four tries and earning two Man-of-the-Match awards during the tournament.