Now the OPML Editor keeps track of every worldoutline that you open so you can easily open them again.
For now you have to run a script to access the feature.
To start you should open a few files either using the Edit button on your Roots page (on the website), or through items on the Bookmarks menu. Each time you open a file it's added to the database at user.opmleditor.worldoutline.history.
Once there are a few items in the history table, run this script: opmleditor.worldoutline.history.buildMenu. A window should open with a menubar object. The menu commands should appear in your local menubar. Select the name of an outline to open it in the editor.
You can copy and paste items from this menu to your Bookmarks menu.