Dear List,
Consider the following MWE:
\startxmlsetups xml:demo:base
\xmlsetsetup{#1}{tag}{xml:demo:tag}
\stopxmlsetups
\xmlregisterdocumentsetup{demo}{xml:demo:base}
\startxmlsetups xml:demo:tag
%\doif{\xmlnamespace{#1}}{o}{NameSpaceO:}
\xmlflush{#1}
\stopxmlsetups
\starttext
\startbuffer[test]
<?xml version="1.0"?>
<root xmlns="NameSpaceA"
xmlns:o="NameSpaceO">
<tag>text</tag>