"O" was a member of the British Secret Service agency, MI6.
O had previously been a champion sprinter, something which was noted in his file, and at some point met the Doctor in one of their male incarnations. During his time at MI6 he was kidnapped by the Master, and was shrunk using a Tissue Compression Eliminator. The Master kept him in a matchbox, and showed him to the Thirteenth Doctor as proof that that he was not the original "O". (TV: Spyfall)