A well-formed XML document can be viewed and edited in <oXygen/> also as a tree of XML elements. This is possible in the Tree Editor perspective available from -> (Ctrl+T) or the toolbar button that provides specially designed views and toolbars and an editable tree allowing you to execute common actions for nodes of a tree like create and delete nodes, edit node names, move nodes with drag and drop.
If you want to be able to edit XML documents that are not well-formed all the time and still have a tree view of the document you should use the Outliner view in the Editor perspective.