The final version of libcelt is bit-for-bit compatible with Opus. Unlike SILK, there was never a "stable" version of CELT (we routinely broke the bitstream on almost every commit), so CELT development continued after the two codecs were merged, right up until the Opus bitstream was frozen.

That makes Opus a strict superset of CELT, and we've stopped maintaining libcelt, so any existing users should switch to libopus.

It actually includes code from Celt and merged with code from Silk (the Skype codec).

Silk is very good at speech and Celt is very good at music. It is like having gears in your car. You need low gears for one mode of operation and high gears for another. So they combined the two into one and made a single codec.

I imagine behind the scenes it has a transition mode from one codec to another depending on the configuration parameters.