I've compiled a version of libopus, opus-tools, openssl, flac, and a few other dependencies that allow you to play Opus files on a Kindle 2, Kindle DX, or Kindle DXG. You will need the Kindle Jailbreak and the KUAL package. It has similar functionality to the built in MP3 player, it reads all OGG files in the /Opus folder and lets you skip a track or stop the player.
Keep in mind that this is just an experiment. The Kindle 2 based devices don't have a lot of CPU power, so they have trouble decoding. You will hear clicks and pops fairly often, and if you have stereo files, you will notice it switching between mono and stereo decoding mode.