Let me preface all this by saying: OOTP was easily my least favorite Potter book. In fact it's one of the most frustrating stories that I have EVER read. It wasn't poorly done or a bad tale, it was just an accurate depiction of a worst-case-scenario that frustrated me to no end.
The movie is a different story.
Up until this weekend, when I finally managed to catch the latest Harry Potter flick, Harry Potter and the Order of the Phoenix, the third film, Prisoner of Azkaban was my favorite. Azkaban to me captured the growing darkness in the wizarding world(which is also well seen and evolving in the two films following it), and it just had a very smart story. To top it off, Azkaban introduced Sirius Black, easily my favorite character in the Potter-verse.
All that to say, I believe that Order of the Phoenix surpassed its predecessor in only one viewing.
OOTP the film manages to capture, in a little over two hours, the heart of the book(which is an intimidating 870 pages), and, for all the shaving of the story that had to be done with the time constraints, seems to lose all that made the book so hard to swallow. We're still presented with a worst-case-scenario: the wizarding community simply doesn't believe Harry and Professor Dumbledore's claims concerning the return of Lord Voldemort. This is a bad situation. Discredited by the media and doubted by his classmates, Harry feels dreadfully alone in his fifth year at Hogwarts, despite those friendships that he has and seems to be forgetting.
The film renders all of this, well, beautifully. The story is not only powerful, but it's just well done. The cinematography is clever and different than in the previous films. The score is phenomenal, and allows one to forget John Williams's absense, unlike that in Goblet of Fire. Even the acting is well done, though several characters' scripts did not allow them the impact that they were capable of(Sirius, Lupin, and Snape, I think, but certainly Ron and Hermione). Imelda Staunton does a particularly fine job as Professor Umbridge, though as her name suggests, she is hardly a pleasant character.
The movie is also full of pleasant sublties pointing towards year 6, that, while readers will recognize them for sure, the sharp viewer also may catch. The sublties of the book that are lost here, really, are the frustrating ones.
This film also shows you why so little was really made of Harry and Cho in Goblet of Fire... again, I accredit this to the time constraints. Frankly, if the film were to be extended at all, I would give the extra minutes first to longer glimpses of the frequently-displayed newspapers and edicts in Hogwarts that the audience's eyes barely have time to scan. The brevity of these shots really proved to be my main gripe against the film, and, to quote Mars Attacks! "that ain't bad".
If you're a stranger to the films(or the books), I certainly would not commend Harry Potter and the Order of the Phoenix to you, just because it is part 5 of 7. There is to much back-story to be understood to really jump in here and enjoy the ride. However, for (most)fans of the books, and anyone who's simply enjoying the films this should be a pleasure. Harry's story begins to follow an even darker path in year 5, but it's necessary and still a joy to travel with him.