BASSenc_OPUS is an extension to BASSenc that allows BASS channels to be Opus encoded without needing an external encoder (eg. OPUSENC). It can be downloaded from the BASS website:

The BASSENC_OPUS.CHM file should be copied into the same directory as this file (BASSENC.CHM), to be able to conveniently access the BASSenc_OPUS documentation from within this BASSenc documentation.