Opus (or other lossy codecs) try to minimize the perceptible loss. The phase can be completely different, so compressing the difference results in the same size as compressing the original... But you'll need to store the opus bit too.

There are several such codecs however, with a lossy and a lossless part. Not because it is easier, but for compatibility. DTS HD Master Audio is one of them: it has a backwards compatible DTS core.