Hugo Oberstein was a German spy living in London, and one of the three most effective foreign agents in Britain according to Sherlock Holmes. He is mentioned in "The Adventure of the Second Stain" and appears in person in "The Adventure of the Bruce-Partington Plans".
When a piece of inflammatory secret correspondence from a foreign head of state went missing from the home of European Secretary Trelawney Hope, Sherlock Holmes named Oberstein as one of the three most likely recipients of the letter, along with Louis La Rothière and Eduardo Lucas. However, Watson's discovery that Lucas has been murdered shifted Holmes' attention to the latter.
After secret submarine plans stolen from Woolwich Arsenal were found on the corpse of Cadogan West, Mycroft Holmes again named Oberstein as one of the only three agents in London capable of handling such an affair, the other 2 are Adolph Meyer and Louis La Rothière. It later transpired that Oberstein, working with Colonel Valentine Walter, was indeed involved in the theft and was in addition West's murderer. Though Oberstein managed to escape to France, Holmes had Colonel Walter lure him back to Britain, where he was arrested and sentenced to fifteen years imprisonment.
- Hugo Oberstein is one of the few minor characters in the Holmes stories who is brought back and used for more important reasons in a second story.
- Hugo Oberstein is presumably named after the German city of Idar-Oberstein.