Android - What can play Ogg Opus on Android?
VLC for Android is reported to support opus format. (src)
According to the Supported Media Formats page Opus is supported natively since Android 5.0, but only in the Matroska (.mkv) container. The Xiph.org wiki and IETF draft state that the mime-type for Ogg Opus files is audio/ogg. To test I encoded a Flac file with opusenc and changed the extension to .ogg before transferring the file to the device, which made it appear and play in Google Play Music, but without metadata like tags and cover art, just the filename.
Edit 2016-05-20: I just encoded¹ a few test files directly to .ogg again and they play fine in Google Play Music². The basic tag fields I use are detected, embedded album art as well as AlbumArt.jpg files work too.
Edit 2016-10-04: I found information about a draft to encapsulate flac-in-mp4 (probably related to Mozilla's efforts to support FLAC in Firefox) as well as opus-in-mp4. I never thought that this would be possible or considered to be implemented, but it appears that some people in the community are working to bring Opus and FLAC to MP4.
- Encoding details: 44.1kHz Flac source, opusenc/libopus 1.1.2 from Ubuntu 16.04, 112 kbps bitrate, file transfer via Syncthing (not required, that's just what I use currently).
- Google Play Music Version 6.8.2817A.2862803. Device: Nexus 5 with CyanogenMod 13.0-20160514-Nightly based on Android 6.0.1.
I've found GoneMAD Music Player and Neutron Music Player support playback of Opus.
Update:
If gapless playback is an important feature to you try XMMS2 or Rockbox.
Both are still in the development stages as far as Android is concerned but they are both worthy of a mention.
XMMS2 is a music player server, so you will also need to download a controller to use it.
Rockbox isn't available via the Play store and it can be a bit tricky to setup so you will have to do your own homework on this one.