StarOffice 5.x had an option to save embedded graphics in the original format, and it is evident from the xml output that OpenOffice maintains knowledge about the original format of embedded objects, even if they're imported from Word or Powerpoint (didn't check further).
It would be nice to have the possibility to Export graphics in their original/native format, e.g. to prevent losses by re-applying jpg compression to a jpg figure. This could be implemented e.g. by an option to the Export dialog, e.g. one that disables the format droplist.