He also represented South Australia at the 2016 AFL Under 18 Championships and was named as the centre-half back in the All-Australian team.
[5] Cox made a case for All-Australian selection during the 2022 AFL season with his ability to shut down some of the best forwards in the competition, he was named in the initial 44-man squad but didn't make the final 22.
[9] He was sent forward the next round against North Melbourne during the last quarter after Fremantle struggled to capitalize on their scoring opportunities, kicking a goal.
[10][11] Brennan made the line-up for Fremantle's opening game of the 2024 AFL season against the Brisbane Lions playing as a key forward.
Unfortunately, Cox suffered a hamstring injury in the final quarter after landing awkwardly whilst contesting for a mark.