HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.xml\UserChoice is responsible for that. If there is no value defined, Directory Opus gets stuck, but only on double-clicking or hitting Enter on file, right-click context menu OPEN (bold, which should indicate default action) works with and without that registry key/value. Explorer seems to have ability to fall back to notepad if there's nothing under that key.
...\User Choice isn't the only key that will work, but it's one of several that will.
Which version of Opus are you using? We made some changes recently which added an extra fallback which Explorer uses and Opus didn't. They should only help with incorrect registry settings, but incorrect registry settings aren't that uncommon so we added it.