Currently SMPlayer has one setting for configuring the buffer for streamed or dynamically downloaded media. (Settings -> Performance -> Cache -> Cache for streams) This is not optimal for listening to audio.
SMPlayer will prebuffer about 20%, which by default is about 400 KB. On the , if I choose to stream Endless Drone, the player will buffer for about 40 seconds. This is not specific to Opus, and will occur with regular audio/mpeg as well. I noticed this with Opus, however, because SMPlayer is very convenient to use, integrated via a right-click menu, since Winamp lacks good Opus support, and those streams also tend to be lower bitrate.
If I adjust the buffer to 4 MB to be more suitable for HD video, SMPlayer will prebuffer 800 KB...
The player could provide 3 fields in Settings, audio/video/default, and choose based on the MIME sent by the server. Default - for content-types it doesn't know of.