That claims they need to be in a MKV container, but I just use the output of opusenc.
EDIT: It looks like TFA says that Android does use Opus:
> Opus is now widely deployed on a large range of platforms and devices (including Android, iOS, and all major browsers) and is exposed to untrusted data.
My tests (on a Pixel running 7.0 and the O beta) have determined that Opus is usually fine as long as the container and file extensions are both Ogg. I have had a few issues with certain files but I'm not sure why yet, I suspect it's album art or something.
I don't believe Android <= 6.0 supports Opus as audio on its own. The CDD initially only required support for Opus in a Matroska container and I attempted to put an Opus-only Matroska file on my phone and none of the players were able to play it. As of either 6.0 or 7.0 (can't remember which), Ogg support is required though.
https://play.google.com/store/apps/details?id=com.jrtstudio....
I reasoned the additional lossy encoding of Bluetooth would be too much with lower bitrates, but now I'm not so sure...