Nothing on the Opus side changed here in a long time. And the Office viewers are the same ones that File Explorer uses. You usually cannot view Office documents without Office installed (or something like QuickView Plus, one of the few other things that can display Office documents).
Get things working in File Explorer, then worry about Opus. The issue is likely not inside Opus or your Opus configuration, unless you can view the documents in File Explorer but they still don't show in Opus.
A long time ago, Open Office included a viewer designed for Internet Explorer, but they abandoned that and it hasn't worked in 10 or more years.
The "Office (Legacy)" option won't help unless you have a very old version of Microsoft Office installed. It won't do anything with Open Office.