"O" was a member of the British Secret Service agency MI6.
According to the Thirteenth Doctor, he was one of their best. O had previously been a champion sprinter, something which was noted in his official file.
However, "O" was ambushed and killed by the Master before his first day, being miniaturised using a new and improved Tissue Compression Eliminator.
The Master kept "O"'s body in a matchbox, which he showed to the Thirteenth Doctor to confirm his identity. (TV: Spyfall)
Not unlike his killer, "O" had short black hair. (TV: Spyfall)