The Great Oppressor was the title of a 20th century Archie Maplin film that the Tenth Doctor said he had seen. It came from a later stage in Maplin's career — a part of Maplin's body of work that the Doctor preferred. (COMIC: Silver Scream)
Behind the scenes
The title is a thinly-veiled reference to Charlie Chaplin's The Great Dictator. For rights reasons, IDW were not able to use Chaplin or his films in the story.