I tried something like this, but that does not seem to work:

  <xsl:template match="ns1:O">
      <xsl:element name="ns1:Q">
         <xsl:attribute name="xsi:type">ns1:Q_Type</xsl:attribute>
         <xsl:apply-templates select="ns1:C" >
      </xsl:element>
  </xsl:template> 


you could do that, or equivalently

I
<xsl:template match="ns1:O">
    <ns1:Q" xsi:type="ns1:Q_Type">
       <xsl:apply-templates select="ns1:C" >
    </ns1:Q>
</xsl:template> 

and you need to make sure that the ns1 and xsl namespace prefixes are
both declared (on your xsl:stylesheet element, usually)


> (sorry, but I am not sure which processor I have, since it is embedded
> in an existing application)

you can ask it.
<xsl:value-of select="system-property('xsl:vendor')"/.
for example

David