Well it's a (nominally anyway, legally this has not, to my knowledge, been tested) patent-unencumbered lossy audio codec from (which I think is really one guy, Monty Montgomery, not 100% sure though), previously best known as the developers of the FLAC lossless compressed format and the Ogg Vorbis (.ogg) compressed format. It's basically the successor format to OGG and Speex, another audio codec, but meant for low-bitrate voice.

According to people who care:

it beats or is on par with the best AAC encoder that consumers can currently use cheaply, the one that comes with iTunes (no kidding). (Though again sample size is not large.)

To answer your question though - no, not only won't it work on all mp3 players, but there are very few client devices that can play it of any note: the foobar2000 music player on the desktop (available for multiple operating systems), and a few other 'power-user' type clients can, but support is not built-in to any operating system (not even Google's Android, even though it has, or had, a semi-close relationship with Xiph iirc). It's possible that the newer Chinese-brand players (FiiO, XDuoo) can play them out of the box but I'm not actually sure. It's also possible that some custom portable music player firmwares, like Rockbox, can play opus on certain devices, but again, I haven't checked and haven't tried this in a while.

Anyway, the point is, that is part of why you were right to say that the death of mp3 is greatly exaggerated. There are plenty of legacy clients that are only compatible with mp3, or that sometimes have trouble with newer formats even if they are nominally supported.