The IETF has standardized the Opus lossy audio codec as RFC 6716. While most audio codecs aim to solve specific problems—relatively high bit-rate music reproduction such as AAC and MP3, low latency voice reproduction such as Speex and the AMR family—Opus provides a "one size fits all" option which, according to its developers, provides best-in-class quality in almost all applications.

The codec was primarily developed by Mozilla and Xiph.org, with contributions from Skype/Microsoft and Broadcom. Opus weds Xiph.org's low-latency high bitrate music-oriented CELT algorithm to Skype's low bitrate speech-oriented SILK. The codec switches between algorithms depending on the bandwidth available, while boasting real-time latencies across the full bitrate/quality spectrum.

The Opus working group's own quality comparison suggests that, between about 12kbit/s and 128kbit/s at least, Opus boasts better quality than any other codec on the market. It loses out slightly at very low bitrates to the AMR-WB and AMR-NB codecs used by GSM telephones, but these codecs require the payment of royalties.

The CELT portions of the codec are covered by patents from Broadcom and Xiph.org; the SILK portions by patents from Skype/Microsoft and Huawei. However, all the patent holders have committed to making those patents royalty-free now that Opus is an IETF standard. The codec also includes an open source, BSD-licensed implementation.

This freedom from royalties is important for one of Opus' motivating usage scenarios. Opus is likely to be made a mandatory part of the WebRTC ("Web Real Time Communication") specification. WebRTC defines ways for browsers to access webcams and microphones to create audio and video streams suitable for building real-time voice and video calling apps that work without any plugins or extra software.

The group developing WebRTC is keen to avoid the fighting about codecs that surrounded HTML5's <video> element. HTML5 doesn't require any specific codec, and while H.264 is supported by most browsers and arguably has most support from media producers, it's not royalty-free, which in particular is a problem for Mozilla. The organization has considered supporting H.264, at least on mobile devices, as a pragmatic necessity.

The flexibility and quality of the codec means that it may see adoption even beyond this role; the low latencies make it attractive for streaming, but the high quality means that even latency insensitive applications, such as music storage, are a good fit for Opus.