Opus-CBCS (Opus Computer Based Conversation System) is a bulletin board system (BBS) program that runs under MS-DOS. It was popular from the mid-1980s through the 1990s.
Opus was created by Wynn Wagner as a FidoNet-compatible BBS program, designed to be compatible with Fido (the program that originated FidoNet) so that sysops of existing Fido boards could easily switch to Opus and preserve all their content, configurations, and user records. Thus, the files used by Opus are almost entirely the same as those of Fido, though over time some of them diverged in later Opus versions (e.g., the conversion of file lists from flat text files to a database format).
Opus rapidly gained popularity by supporting everything Fido did, plus some enhanced features such as the use of color in menus (for users with terminal programs supporting one of several protocols that supported this), and some new and improved data transmission protocols which made more efficient use of modems.
Later on, though, it lost ground to other BBS software which had greater support for such things as online games and multi-user chatting. There was a long delay between the release of versions 1.20 and 1.70 (they skipped the version numbers in between), with the only word on possible release dates being "When It's Ready", and many people switched away during that time, leaving a few hard-core Opus fans sticking with it.
Opus was one of the few FidoNet-capable boards that supported built-in network mail (and Echomail, the FidoNet answer to forums or newsgroups); other BBS software generally required an add-on front-end to support these features. Many Opus sysops, however, still used front-ends on their boards to do netmail/echomail, since those programs had more features in this area.