Similarity and/or Diff of two ODT documents (Linux)

Did you try the built-in compare functionality? Edit -> Track Changes -> Compare Document....

If you just want a textual diff, your best bet is probably to convert both documents to plain text, then run a regular diff on them. You will have to figure out how to normalize linebreaks though, otherwise the diff will not be very useful.

I haven't tried them myself but this pair of tools - odt2txt and oodiff - looks promising.