What is an oxymoron in The Hound of the Baskervilles?

An oxymoron is a literary device in which two opposing concepts or words are juxtaposed against each other. 
We can find a great example of an oxymoron in chapter eight of The Hound of the Baskervilles, "First Report of Doctor Watson."
On October 13th, Watson writes to Holmes from Baskerville Hall to describe how being there is like leaving behind all traces of contemporary England. He describes the graves that are scattered throughout the hillsides and the gray stone huts that are there too. He goes on to report the facts concerning Sir Henry Baskerville and to mention the escaped convict who is prowling the moor.
In the opening passage of his letter, we can find the oxymoron. Watson states that the longer he stays there, the more the spirit of the moor sinks into his soul—both its vastness and its "grim charm."
"Grim charm" is an oxymoron which juxtaposes a positive quality (charm) against a negative descriptor (grim). Watson is suggesting that while the moor is initially frightening, one can become accustomed to it.