Are there any iPhone apps which can display/edit OPML documents?
Depends on what you want to do exactly but since OPML is just another Markup Language you can use any text editor or maybe one that's also able to highlight XML. You could also try online tools such as google docs, tho I couldn't myself. Here's another way to read only OPML using iGoogle.

I'll also give a shot and guess what kind of OPML you'd be using and suggest this app.