You can now listen to quite a bit more of the OOP movie soundtrack here (although it's still just snippets). It was written by Nicolas Hopper, and as a whole it's much different than either the John Williams three (Stone, Chamber, and Azkaban) or Patrick Doyle's Goblet of Fire soundtrack (which I thought was pretty much just a more intense version of John Williams).
There are still bits of a slightly darker "Hedwig's Theme" (the main theme throughout all the movies), most noticeably in "Another Story". But that's the only part that really hits you over the head from the other movies. The rest seems very fresh and new.
I really like the (for complete lack of a better word) staccato-y-ness of it all--my favorite example (which you can hear right at the beginning on the Order of the Phoenix movie website, but not in this set of samplers) is very plunkety-plunkety, kind of like raindrops. Back to this newest sampler though, I also like what he's done with the Dumbledore's Army scenes ("Dumbledore's Army" and "The Room of Requirements") as well as with the Ministry of Magic scene ("The Hall of Prophecies", "The Ministry of Magic", and "The Death of Sirius"). Further I like the addition of the electric guitar ("Fireworks"), as I think it kind of helps age everything up a bit (i.e. the characters are getting older, the music should reflect that).