A software package, used on the Macintosh platform with operating systems prior to OSX, for connecting sequencer software to MIDI interface hardware. (OMS originally stood for "Opcode Music System", after the Opcode Systems company which wrote it.Later, they changed the acronym to "Open Music System" after Opcode opened the specification up to third-party developers.) The OMS system allows the user to build a description of what type of MIDI interface hardware is being used and which synths and other devices are using which outputs and channels. OMS then provides this information to the sequencer software, and the sequencer can address the MIDI-using devices without regard to what type of interface hardware is being used.OMS was, for almost a decade, at the center of a long and strange odyssey for Mac developers and users. The software has been unsupported since 2000 when Gibson purchased and effectively disbanded Opcode, but it remained in use because legal restrictions at the time prevented Apple from developing a replacement. Existing sequencers that relied on OMS had to keep using it, even though there was no support, because a practical alternative did not exist (porting to FreeMIDI would have been a significant effort).

Apple finally worked out its legal problems, and the OSX operating system introduced a new MIDI routing system called Core MIDI, with the help of some ex-Opcode developers. OMS remains in use on some old Macs still running OS9, but it was not ported to OSX and it is now fading into history.

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