XML files are opened in Oxygen because Internet Explorer uses the Windows file associations for opening files and you associated XML files with Oxygen XML Editor/Author in the installer panel called File Associations. This installer panel displays a warning above the XML file association that XML files will not be viewed correctly in Internet Explorer if you associate them with Oxygen XML Editor/Author.

For viewing XML files in Internet Explorer again you have to associate XML files with IE by right-clicking on an XML file in Windows Explorer, selecting Open With -> Choose Program, selecting IE in the list of applications and checking the checkbox "Always use the selected program". Also you have to run the following command from a command line:

wscript revert.vbs

where revert.vbs is a text file with the following content:

  function revert()
    Set objShell = CreateObject("WScript.Shell")
    objShell.RegWrite "HKCR\.xml\", "xmlfile", "REG_SZ"
    objShell.RegWrite "HKCR\.xml\Content Type", "text/xml", "REG_SZ"
  end function
                        
  revert()

Associating an extension with <oXygen/> in Eclipse 3.4+ requires three steps:

1. Associate the .ext extension with the Oxygen XML Editor: go to Windows -> Preferences -> General -> Editors -> File Associations, add *.ext to the list of file types, select *.ext in the list by clicking on it, add Oxygen XML Editor to the list of Associated editors and make it the default editor.

2. Associate the .ext extension with the Oxygen XML content type: go to Windows -> Preferences -> General -> Content Types and for the Text -> XML -> oXygen XML content type add *.ext to the File associations list.

3. Press the OK button of the Eclipse preferences dialog.

When a *.ext file is opened the icon of the editor and the syntax highlight should be the same as for XML files opened with the Oxygen XML Editor.