It's an original riff called "Miss Gulch's / Witch's theme" by Herbert Stothart, who won an Oscar for the film's score. Also, according to [Wiki][1]:

>This repeated seven-note motif is actually a "crippled" variation (inverted and compressed in range) of the musical figure for "We're off to see the Wizard".

Here's an interesting [article][2] ([archive.org copy](https://web.archive.org/web/20150923064438/http://news.moviefone.com/2013/09/20/whats-the-most-important-song-in-the-wizard-of-oz/)) about the music in the film (this riff included).


  [1]: http://en.wikipedia.org/wiki/Musical_selections_in_The_Wizard_of_Oz#Music_in_the_film
  [2]: http://news.moviefone.com/2013/09/20/whats-the-most-important-song-in-the-wizard-of-oz/