Describe the file format
Opus is an audio codec that is meant to replace the Vorbis codec. Currently, convert.to.it can convert OGG Vorbis files to OGG Opus files, but not the other way around.

Provide reference implementations
FFmpeg should be able to do this in theory, but I'm not sure if a web build can.

Do you plan on implementing this yourself?
No, I don't understand the FFmpeg handler well enough to do so.

Additional context
If FFmpeg isn't able to convert from Opus files, a web version of libopus (such as this one) could be used instead.