To define default mappings between prefixes (that you can use in the XPath toolbar) and
namespace URIs go to 
XPath Options
preferences panel and enter the mappings in the Default prefix-namespace
mappings table. The same preferences panel allows you to configure the default
namespace used in XPath 2.0 expressions.
Important: If you define a default namespace, Oxygen XML Editor binds this
namespace to the first free prefix from the list: default,
default1, default2, and so on. For example, if you define
the default namespace xmlns="something" and the prefix
default is not associated with another namespace, you can match tags
without prefix in an XPath expression typed in the XPath toolbar by using the prefix
default. To find all the level elements when you define a
default namespace in the root element, execute this expression:
//default:level in the XPath toolbar.