After upgrading OS X and Oxygen XML Editor, it is no longer associated to the appropriate file types (such as XML, XSL, XSD, etc.) How can I create the file associations again?
The upgrade damaged the file associations in the LaunchService Database on your OS X machine. You can rebuild the LaunchService Database with the following procedure. This will reset all file associations and will rescan the entire file system searching for applications that declare file associations and collecting them in a database used by Finder.