Users of oXygen and Tower, a GUI git client (https://git-tower.com), may find this gist useful:

  https://gist.github.com/joewiz/788e66a6f526b2b8d7c065aa69cdfbe2

It explains how to register oXygen XML Editor's Diff & Merge Tools with Tower (on macOS), so that when you want to see the diff of a changeset or resolve a conflict, oXygen's diff tools are opened.  The gist also contains a screenshot showing how Tower displays the oXygen-related preferences and functionality.

Thanks to Octavian for the pointers to the relevant documentation.  Comments welcome.

Joe