After beginning his career with Cork City, the 23-year-old completed a move to Premier League side Crystal Palace in 2021.
O'Brien spent time out on loan at Swindon, before he joined RWD Molenbeek on a temporary deal for the 2022-23 season, helping the Belgian side win promotion to the top flight.
The defender went on to join Lyon in a €1m move last summer, leaving Palace without making a single appearance for the first team.
However, O'Brien experienced no lack of game time in his first season in France, starting all 32 of his competitive appearances.
Brentford leading Premier League chase for O'Brien
O'Brien helped the club achieve European League qualification via a sixth-place finish as well as playing his part in their run to the Coupe de France final, where his goal proved to be fruitless in a 2-1 defeat to Paris Saint-Germain.
Having impressed in his first season as a Lyon player, O'Brien is now being linked with a move away despite being under contract until June 2027.
According to Foot Mercato, Brentford are showing a keen interest in signing O'Brien ahead of the 2024-25 season.
The report claims the Bees have already made a contract with Lyon and the player to discuss a potential transfer this summer.
While Brentford appear to be leading the race, they could face competition from at least two other Premier League clubs, with Everton and Wolverhampton Wanderers keeping tabs on the situation.
Lyon's willingness to listen to offers was demonstrated by the fact they decided against risking the centre-back for Wednesday's friendly against St. Pauli.
Pierre Sage's side fell to a narrow 1-0 defeat in the pre-season fixture, as O'Brien watched on from the bench as an unused substitute.
Why do Brentford want a new centre-back?
Brentford boss Thomas Frank can currently call upon Nathan Collins, Kristoffer Ajer, Ethan Pinnock, Zanka, Ben Mee and Ji-soo Kim as his central defensive options.
While the Bees may have a number of options, they will be wary of the fact that Mee and Zanka are out of contract at the end of the 2024-25 season.
Brentford also struggled defensively last term as they shipped 65 goals in 38 matches - 19 more than the 46 they let in during the 2022-23 campaign.
The Bees seemingly view O'Brien as someone who could shore up their backline and help them produce a better performance than last season's 16th-place finish.