Agent O (shortened to Oh or just simply O) is an agent of the MiB, who appears in Men in Black III. She is a veteran agent, in the film becoming chief of the MiB after Zed's passing, who was a secretary back in the 1960s to Zed's predeceesor Chief X. Agent O had a love affair with Kay during this time, but they know their work comes first so they ended their love story. Still, Kay has clear interests in her.
She was portrayed by Emma Thompson in the present day, and by Alice Eve in the 1960's scenes of MIB 3.