Docbook Viewer for OS X
I don't think there is a DocBook viewer for OS X.

The workaround seems to be to convert the DocBook document to another format using a converter.  Here are some suggestions:


dblatex: Can be used to convert DocBook to PDF.  It is free, and installable using Fink.  However, if you are already using Brew you might mess up your system if you start using another package manager.  An alternative would be to use dblatex on a Linux VM, but that would be a great deal of work if you don't already have one set up.
OxygenXML - a commercial product that can read DocBook XML.  It has a 30 day free trial period.   I tried it on my DocBook document, but it was not well formed and Oxygen couldn't parse it.


So in the end, neither of the two options worked for me, but they might for you.