Well, the way WoG displays movies is actually using chapters, that is, the movies are accessed by chapter's island.xml, rather than the level itself. If you look at the island.xml page, you'll see that each <level> tag has a bunch of attributes, one of them being a cutscene attribute. You can read there how it works.

I might find some time to look into this more and see exactly what is required for movies to work (i.e. they may need to be specified by resources.xml, I'm not sure). However before you go adding movies to the ends of your levels, make sure that you're comfortable with XML and that you know how chapters work.

I used new custom movies in Earth Of Goo! You could download that (Link in my signature) and see how I did it. Not sure you'd understand it, but it worked! You just have to make sure the movies are converted to .bintl format after making them in Goovie and there's a button to do that in Goovie I believe.

OCD's are in the island file as well, but you can use them in custom levels. The problem here is a bug in GooTool itself: the cutscene tag is ignored in the mod manifest. You need to merge it in with XSL, which is a pain for a single level.