- "Alex, you were so busy at Quantico trying to get redemption for killing your father that you never saw me making you into a scapegoat and you make a great scapegoat, Alex, because you believe in this country even after it thought you were a terrorist. You still love America, and you think it loves you back, but it doesn't. Never did and now you won't be there to see it burn."
- —Liam O'Connor to Alex Parrish
Liam O’Connor is a jaded, tough FBI agent who begrudgingly takes on the responsibility of teaching the new class of NATS at Quantico. And that's all thanks to the Deputy Director -- and his former flame -- Miranda Shaw, who pulled strings to help him keep his dignity and his pension. Unbeknownst to Miranda, however, Liam is using an undercover agent of his own to run his own secret side operation. Perhaps Liam isn’t helpless and innocent after all. Are there skeletons in his closet from his FBI past that make him play his cards so close to his chest? What kind of secret does he have and what does it have to do with Alex Parrish? In the future, Liam is in charge of the hunt against the terrorists responsible for the bombing at Grand Central Station. Convinced of Alex’s guilt, Liam says that he knows Alex better than anyone else, but could his understanding of Alex be more than just professional?
Liam O'Connor is a seasoned FBI agent who had a fling with Miranda Shaw in the past. Years ago, he and Alex's father had graduated from the FBI academy together and one of their first undercover operations in Omaha was a blown sting killing over 200 people. Years later, he worked a sting operation against some militia groups in Chicago. Just before the case went south, ending in a shootout with three confirmed casualties including Drew's fiancee and unborn child, he had told Ryan that he would take the blame and that Ryan would be his replacement. However, it was later revealed that Ryan took the blame for the incident, later losing his position within the Bureau.
One year prior to the Grand Central bombing, it was revealed that Liam was demoted from his position within the Bureau. However, Miranda pulled strings to allow him to work as an instructor at the Academy. During his time at the Academy, he had a one-night stand with one of the recruits, Alex on New Year's Eve. Although, Miranda discovered the incident, she was shortly dismissed prior to pursuing the misconduct case with Liam. Sometime later, Liam was promoted to the position of Assistant Director, replacing Miranda's former role at the Academy.
Months after Alex's cohort graduated from Quantico, Liam was assigned to work in a field office based in Washington. After the Grand Central bombing occurred, Alex was considered a main suspect in the case. As a result, he pursued her until she managed to clear her name. Three months after the Command Center bombing occurred, it was revealed that he testified against Elias Harper in the congressional hearing case. Later, Miranda discovered that he was the true terrorist behind the two bombings in New York, right before he shot her. Subsequently, it was revealed that he used hidden cameras and tracking devices to plan the bombings and gathered intelligence to blackmail Elias and frame both, Alex and Simon. His objective was to destroy the Bureau before the organization got an opportunity to destroy the country. Initially, he tried to frame Miranda, but Alex and the others discovered his plan to plant a bomb at the FBI Academy, the day the new cohort were graduating. However, he was unsuccessful as the bomb was later defused by Simon's actions. Prior to that, he was killed by Alex and Ryan.
- He has a daughter named Louisa O'Connor.
- He was an FBI instructor at Quantico.
- He is the second main character to die. The first was Natalie Vasquez.
- He was a former romantic interest of Miranda Shaw.
- Years before the events of Run, he graduated from Quantico along with Alex's dad, Michael Parrish.
- In Right, it was revealed that he was on an undercover mission in Omaha with Michael and a local militia group which later ended in a failed sting resulting in the deaths of over 200 people.
- In Over, it was revealed that Liam applied for an immediate transfer request out of the field office that he formerly worked in, sometime in 1993 to his boss at the time, Michael Parrish. Presumably, the request was made after the failed sting at Omaha.
- In Answer, it was revealed that Liam had a sting operation that targeted certain militia groups. As part of a sanctioned mission by the Bureau, a number of Panther Oracle AR-15s guns were given to a militia organization in Chicago. Prior to arresting members of the organization, there was a shootout at the Chicago rapid transit train between the militia group and the law enforcement officers from the Chicago Police Department. As a result, three casualties were recorded.
- In Right, it is shown that he is the mastermind terrorist, who is responsible for planting the bombs at the Grand Central station and the Command Center bombing. In addition, it was revealed that Liam blackmailed Elias into helping his bombing plans as well as killing the following victims; Clayton, Diane, Natalie and Drew.
- Towards the end of Yes, Liam's motivation for the bombings is revealed along with his connection with Claire Haas. He is later killed in the episode by Alex and Ryan.