The 20-year-old came through the ranks at Aston Villa's Academy, before making her senior debut for Coventry United in 2018.
She made 12 appearances for the West Midlands side in all competitions in 2019/20, scoring five goals, and then featured 10 times the following campaign, finding the net once, ahead of making the move to the Foxes.
On 7 March, 2021, O'Brien opened her account for Leicester City with a brace of well-taken goals in a 7-0 win at London Bees in the Barclays Women's Championship.
In the 2020/21 Vitality Women's FA Cup Fifth Round clash with Manchester United, meanwhile, the attacker scored as Leicester beat WSL outfit Manchester United 3-2 away from home to progress into the quarter-finals.
A consistent performer in her debut WSL season, O'Brien scored the winner in LCFC Women's meeting with Brighton & Hove Albion in January 2022 and also found the net in the Continental Tyres League Cup - helping City to success over Durham in November 2021 - and Vitality Women's FA Cup this campaign, netting the equaliser in victory at Tottenham Hotspur.
Those goals saw O'Brien finish last term as joint-top goalscorer and led to her winning LCFC Women's 2021/22 Goal of the Season award, as well as taking home the Player of the Season prize, as voted for by the fans.
The attacker made her international bow for England Under-23s in Feburary 2022, featuring from the bench in two friendly fixtures against France.
After an injury which keep her sidelined for the majority of the 22/23 season, the forward has returned for the new campaign in fighting fashion and has been utilised as a winger by former Everton manager Willie Kirk.