You can use WinFF, a gui front end for ffmpeg
sudo apt-get install winff
Open WinFF and then close it again. Open the preferences file located at
~/.winff/presets.xml
add the following section then save the file and reopen WinFF
<opus>
<label>Opus</label>
<params>-acodec libopus -b:a 256k -vbr on -compression_level 10</params>
<extension>opus</extension>
<category>Audio</category>
</opus>
That is set to 256k vbr and ffmpeg seems to convert to 48000Hz by default
If you choose the Convert to: dropdown as 'Audio' in WinFF, the Preset: section will then have an Opus setting to select
You can make as many of the above presets as you like for different bitrates etc, just the outer tags need to be renamed and the label changed to whatever you want.
The full list of Opus options can be found using
man ffmpeg
To inspect the output install the following
sudo apt-get install opus-tools
then use
opusinfo foo.opus
to look at what you have created.