Take a look at this website, which covers all things Oliver. In 1958, the Oliver line had a 6HP, and it is possible that your 5.5HP is based on an earlier Oliver, or it is a slightly "de-tuned" version on the 6HP. I’m not sure exactly what Oliver did during its brief stint making the Wizards, but when Mercury was making Wizards, their Wizards were often lower HP versions of the Mercury lines, or somewhat hybrids put together from various Mercury models.
This site has numerous Oliver manuals, as well as the owner’s contact information. I bet he can help you figure out what you’re dealing with.
http://oliveroutboardfixation.com/?page_id=374
Good luck!
JP