[3][4] She started to compete at the age of 12 by participating in online qualifiers for competitions such as Wodapalooza.

[3] She joined HWPO in January 2022 to be trained by the former champion Mat Fraser.

[3] At the 2021 CrossFit Games, O'Brien competed in the elite women division for the first time.

[8] She eventually finished 7th overall and was named Rookie of the Year, thereby becoming the first athlete to win the accolade while still eligible for the teen divisions.

[2] At the 2022 CrossFit Games, O'Brien finished in second-place after six-time winner Tia-Clair Toomey, which made her the youngest-ever person to reach the podium at the CrossFit Games.