The ex-Crystal Maze host told DS that he had recorded his dialogue for the part of SS head Heinrich Himmler in the animated puppet comedy some time ago.
O'Brien said: "I have no idea what's going on, or what the storyline is, how it's shaped up. I just enjoyed playing a horrible Nazi. We all like to play a bit of a nasty Nazi from time to time.
"We might pretend that we don't want to, but yes we do. So from that point of view - from a childish point of view - I enjoyed myself. It was just a job to do silly voices really."
Other stars involved in the project include Ewan McGregor, Alan Cumming, Richard E. Grant, Richard Griffiths, Stephen Merchant and Sanjeev Bhaskar.
> DS on set: 'Jackboots On Whitehall' preview