[1] He first made state selection for New South Wales in the 2018 men's youth eight which contested and won the Noel F Wilkinson Trophy at the Interstate Regatta within the Australian Rowing Championships.

[3] In 2021 O'Shannessy moved into the New South Wales senior eight to contest the King's Cup at the Interstate Regatta within the Australian Rowing Championships[4] In 2022 and 2023 O'Shannessy rowed in the New South Wales eights which won the King's Cup.

[6] O'Shannessy made his Australian representative debut in a coxless four at the 2019 U23 World Rowing Championships in Sarasota Florida.

[7] The eight won through their repechage to make the A final where they raced to a third place and a World Championship bronze medal.

[7] In March 2023 O'Shannessy was again selected in the Australian senior men's sweep-oar squad for the 2023 international season.