I have RAW-Image files of Pentax cameras with the extension PEF. When I set Photoshop as the standard program (Setting-->File Types-->System File Types...) it works only one time, the next time the file is opened by the standard Windows Foto Viewer, of cause with the message, that it can not read it until I install additional codecs. What can cause this behaviour?
If that isn't working, use the Windows right-click method (right-click a file, Open With > Choose Default Program), or the Windows Settings > Apps > Default Apps UI. Those generally override everything else.
I've found that some programs need to be running as Administrator to change file associations. Perhaps pressing the "Admin" button on the Operations toolbar at the top will allow Opus to change the setting permanently? The ways Leo suggested are the sure-fire ways to achieve this though.
Thank you Leo! Your programm is so great, that I forgot, that Windows itself has a feature for it. That works!