There a few Oubapian works not in the 4 Oupuses (Oupi?). I should have a review of Trondheim and Menu's "Moins de quartre secondes a vivre" up at some point. Trondheim's Mister O is constrained, as is Ayroles "Jean qui rit Jean qui pleur". [Links added.]
And while these works are French in origin, I don't really have that excuse, since the latter two are, in fact, wordless. (I've even read Mister O!)
Derik goes on to mention a work which, while not by a member of the Oubapo, is (based on the description in his review) certainly under a Oubapian-style constraint, NogegoN by Luc & Francois Schuiten. He says of it that "It's technically well done but also a bit soulless, lacking in expressive emotion. All in all a very interesting example of a formally constrained comics work." And his review certainly makes it sound quite interesting.
For that matter, there's also some work in English which Derik didn't mention but which he might well have. For example, there's Kenneth Koch's The Art of the Possible: Comics Mainly Without Pictures, which I am only slightly familiar with, but which is certainly 'Oubapian' in spirit. (Derik's review of it is here.) In fact, one of Matt Madden's Exercises is a homage to Koch's book, so I really ought to have thought of it.
And while I have no reason to think he's ever heard of the Oubapo (though of course he might well have), a great deal of Alan Moore's work is formally inventive in ways which are Oubapian in spirit, from many of the chapters in V for Vendetta to much of his recent ABC work (particularly many different issues of Promethea, and his Eisner-award-nominated "How Things Work Out" from Tomorrow Stories #2).
And having mentioned all those, I suppose people will bombard me with questions about omitting all sorts of other people and books (Chris Ware? Art Speigleman? Will Eisner?), to say nothing of a large number of shorter Oubapian works. So I'd better quit while I'm behind, surrender, and say: I was wrong.
But while I am clearly guilty of intemperate phrasing -- chalk it up to extreme (but, I think, warranted) enthusiasm -- I think I was also getting at something. I'm not quite sure how to phrase it -- first self-consciously Oubapian masterpiece in English, perhaps? But Matt Madden has done something extraordinary, not only due to the quality of the work -- although because of that too, most importantly -- but because it is pathbreaking as well. So even if there are works I forgot, and others I am ignorant of, I nevertheless renew my suggestion that you check out Madden's new book.