Hogwarts didn’t change much in Order of the Phoenix; I was able to knock out the major changes in no time.
GOF on the left, OOTP on the right, and you can slide to compare:


As you can see, the south side of the quad building was changed. (The poor little GOF version only lasted one film!) I suspect this was because the quad floor was raised quite a bit in (I believe) GOF. The other change noticeable from that view was the addition of spires at the corners of the clock tower courtyard, on the far left.
Here’s another comparison, with SS on the left and OOTP on the right, to show how far we’ve come in the first five films:


(The SS landscape has been hidden to make it a “fair” comparison with the OOTP version, which doesn’t have any landscape yet.)
Anyway, back to GOF vs. OOTP:


We can see here the other big changes: the steepening of the spires at the viaduct entrance, the addition of a couple new spires, and the addition of Snape’s window, in the dungeon level just to the right of the stone bridge. These tweaks were all featured in a single shot that pulls out of Snape’s office, through the window, and up into the snowy sky above Hogwarts, past the steep spires.
Otherwise, Snape’s window is pretty hard to glimpse; it’s basically out of sight at the Warner Bros. Studio Tour, and there are no other exterior shots of it in the films that I can think of. So here’s a closer view of my recreation:

At close range like this, you can see that the tracery on some of the windows is pretty flat. But I only have so much time to devote to this project, and I’m already drowning in polygons.
Anywhoodle, let’s wrap up with another SS vs. OOTP comparison, showing the whole castle.


Same castle, but…not! Which is kinda the whole point of this project, I suppose, haha.
By the way, I’ve been careful with my phrasing in this post, because these are not ALL the changes in OOTP. The stone circle by the wooden bridge mysteriously disappeared (only to reappear in future films), but I haven’t built the stone circle yet so there was nothing to remove. I also may build Hogsmeade Station when I build the environment, and that got a brand-new design and location in this film.