Agent O appears in Men in Black III and Men in Black: International. She is a veteran agent of the organisation, becoming chief of the New York branch of the MiB after Zed's passing, and was a secretary in the 1960s to Zed's predeceesor Chief X. Agent O was previously romantically involved with Agent K, but at the present time, this has since ended.
She was portrayed by Emma Thompson in the present day, and by Alice Eve in the 1960's scenes of MIB 3.
Agent O started her career in the Men in Black as a secretary to Chief X, and moved her way through the ranks as the years passed, eventually becoming head of the New York detachment of the MiB by 2019. Not much is known about her early life, though she did enter into a relationship with Agent K at some point in the late 1960's, and possibly continued this for some time. As of the present, this is no longer the case.
Agent O is a stern, crafty, and intelligent woman, with years of experience under her belt and commands the respect of the fellow agents with whom she operates with. During the events of MiB: International", she was impressed by the tenacity of a young woman named Molly Wright, recruiting her into the ranks and assigning her to London, pairing her with Agent H and serving under High T.