The XSLT preferences panel is opened from menu .
If you want to use an XSLT transformer implemented in Java different than the ones that ship with Oxygen XML Author namely Apache Xalan and Saxon all you have to do is to specify the name of the transformer's factory class which Oxygen XML Author will set as the value of the Java property javax.xml.transform.TransformerFactory. For instance, to perform an XSLT transformation with Saxon 9.3.0.5 you have to place the Saxon 9.3.0.5 jar file in the Oxygen XML Author libraries folder (the lib subfolder of the Oxygen XML Author installation folder), set net.sf.saxon.TransformerFactoryImpl as the property value and select JAXP as the XSLT processor in the transformation scenario associated to the transformed XML document.
The XSLT preferences are the following: