Opus ][ - The Source contains nearly all the source code written by The Byte Works. There were two main exceptions – Talking Tools (developed by First Byte) and ORCA/Modula-2 (developed by Peter Easdown).

Peter Easdown has now graciously made the source code available on GitHub. The compiler itself is based on the Niklaus Wirth single-pass Modula-2 compiler (written in Modula-2) with modifications to generate 65816 code and support for the IIgs Toolbox and GS/OS.