O (Spyfall)
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"O" was a member of the British Secret Service agency, MI6.
When UNIT went to MI6, The Master was present and was the one who received the message from Missy, however, he was pretending to be O the whole time.
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 by the Spy Master before his first day and miniaturised using a Tissue Compression Eliminator. The Master kept his body in a matchbox, which he showed this to the Thirteenth Doctor to confirm his identity. (TV: Spyfall)
