Oliver Haddo was one of the black druids that kidnapped Mr. Gaunt. He replaced his regular chauffeur, Mr. Senna. Mr. Gaunt did not notice the suspicious behavoirs of his driver until it was too late. Haddo told Mr. Gaunt he was an old friend of the chauffeur and deceived him into a false state of safety.
When Mr. Gaunt had asked to be dropped off at the next corner, Haddo agreed. However when the time came he kept driving to the location of the abduction. Haddo and several other men took him to a warehouse and imprisoned him there.
It was revealed that he was in league with Virgil Macreeby and was one of the closest members in contact with him. In the climax of the book, McCreeby threatened to harm Mr. Gaunt simply by calling Haddo through sattelite phone. Haddo had filmed a video of Mr. Gaunt to prove his captivity, but in this mistake, it gave clues to the location of where he was held.
He was one of the main guards of Mr. Gaunt's prison cell. Sometimes, he would keep the keys on his belt, but other tiems they were concealed of the hook or a back door. Part of the reason for the relaxed security was due to their trust in McCreeby and the fact that there was an exorbere guarding the compound.
Eventually Layla Gaunt found Haddo and turned him into a rare scorpion or a soricomorph.