Originally Posted by JSWolf
You should be using Kindlegen and not KindlePreviewer to convert.
They do the same thing, Jon. Dragging an ePub/OPF/XHTML file onto KindlePreviewer works just the same as as feeding them to kindlegen (because kindlegen is embedded in KindlePreviewer). The only thing you can't do when converting using KindlePreviewer is choose your compression level.
@AThirstyMind: are you using a recent version of KindlePreviewer? I don't think it worked with OPF files until v2.4. Also, you must leave the OPF file where it is in relation to the rest of the ebook's files, so the Previewer can find them (which of course implies that the hrefs in the OPF file accurately point to the rest of the files). And also, the OPF file can't already be zipped up in an ePub archive. If you have an ePub—use