You get more specific help messages by putting in the file you want to convert from and to, and then -h. At first I couldn't quite figure out how to specify an OEB file, because I wanted to convert from HTML to OEB, but finally I figured out that I could just specify a directory and it would convert to OEB. So I got help converting from HTML to OEB by doing:
ebook-convert infile.html oeb_directory -h
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