oXygen 8.2
supports both DocBook 4.4 and 5.0.
According to Norm
Welsh DockBook5 Definitive Guide [quote/unquote]: "The normative schema for DocBook V5.0 is the RELAX NG Grammar with its
Schematron annotations. The only reason to use a DTD with DocBook V5.0 is if
your editing environment (or other tool) requires one, for example, for syntax
directed editing."
The sample provided with oXygen 8.2 has this
declaration:
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V5.0//EN"
"http://www.oasis-open.org/docbook/xml/5.0/docbook.dtd">