Opus is an open free lossy audio codec developed by IETF. Opus has very low algorithmic delay and high compression rate compared to MP3, Vorbis, and HE-AAC, so it's very suitable for Internet real-time interactive application. Most of the time, Opus is used in Ogg or WebM file, and sometimes, it's used in standalone .opus file. If you want to play such files, your player should come with an Opus decoder. Faasoft Audio Converter has an built in Opus decoder, you can use it as a free Opus player.