O'Sullivan first played Gaelic football to a high standard as a schoolboy at Pobalscoil Chorca Dhuibhne. He was part of the school team that won back-to-back Hogan Cup titles in 2014 and 2015, while he was also part of four Corn Uí Mhuirí-winning teams.[2][3] At club level, O'Sullivan plays with Dingle. After a Kerry MFC success in 2014, he has since gone on to win numerous West Kerry SFC titles, as well as a Kerry Club FC title in 2015.[4][5]
Following his first season with the under-21 team, O'Sullivan was drafted onto the senior team in 2016. He won the first of seven Munster SFC medals in eight seasons that year.[8] O'Sullivan was at left corner-back when Kerry were beaten by Dublin in the 2019 All-Ireland final replay, but ended the season with his first All Star.[9] He won three consecutive National League medals from 2020 to 2022, having earlier won his first title in 2017. A second All Star followed in 2021, before O'Sullivan won an All-Ireland SFC medal after a defeat of Galway in the 2022 final.[10]