Ah well, one major point of improvement is the container specification which is part of epub. As far as I know, oebps didn't have a container spec. This makes distributing epub based ebooks a little easier. Beyond that, I can't really give you a detailed answer, as I'm not too familiar with the details of either the oebps or the epub spec. Personally, I view epub as an evolution of oebps, not a radical new format. In fact at the moment, most of my personal collection is in oebps1-ish form
