I tested importing OGG files on my MacbookPro with Mojave 10.14.6 and with both 2.3.2 and the latest alpha build I have for the upcoming 2.3.3 and OGG imports work just fine.
There’s also an on-line tool here that can give you information about the format of audio files: https://mediaarea.net/MediaInfoOnline
If that says that the file is “opus” encoded, then you will need to use something other than Audacity to convert it into another format.
I managed to import an Opus file - following the test steps in this bug (relating only to debug Audacity versions): https://bugzilla.audacityteam.org/show_bug.cgi?id=1945
ASSERT (in debug builds only) importing opus format
My mistake. I incorrectly thought that the problem with opus was export and import. I’ve not tested on macOS, but on Linux I can import opus files without any problem.