Apple - Can iPhones play Opus files?
Update Nov 2017:
Opus is now included in iOS 11.
iOS 11 and Android Spearhead Universal Support For Opus Audio Codec, MP3’s Overdue Successor
Judging by this conversation on the Apple forums, it seems unlikely an iPhone can use the format directly.
Since Opus is not compatible with most popular players and devices so far, it's absolutely impracticable, no matter directly play Opus files on Windows Media Player, Android, PS3, Xbox 360 and iTunes or sync Opus to iPhone, iPod, iPad via iTunes, or edit these Opus in iMovie and QuickTime.
Under the circumstances, the best solution is to convert Opus to MP3 and other more compatible audio formats in order to play or edit Opus in the above popular players and devices.
One of the posters mentioned that Adapter (freeware) can convert then to a playable format, MP3, AAC etc.
Yes, apps on iPhone can play .opus files, but you'll still have to use a 3rd party player on macOS to play the file. That just means that Apple's QuickTime still doesn't support the format as of January 2, 2019.
This format is used in common apps such as WhatsApp to communicate audio recordings between users. Support for this codec has been present in some common (and awesome) A/V software like VLC for several years while some of the biggest players in the hardware/software game are still only adding rudimentary support. There are conversion utilities/applications readily available on the web - just do a simple google search to find what you're looking for.