You can play them in VLC, but a few years ago I remember that Opus wasn't recognised by Android's "media service" (meaning the native players couldn't even detect the existence of Opus files). It's probably changed since then though.

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.