A Well-Formed XML document can be viewed and edited in Oxygen XML Developer also as a tree of XML elements. This is possible in the Tree Editor perspective, available from . The Tree Editor provides specially designed views, toolbars and an editable tree allowing you to execute common tree actions like create/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 and still have a tree view of the document you should use the Outline view in the Editor perspective.