omni mode
A MIDI reception mode in which a module responds to incoming MIDI channel messages no matter what their channel.

OMS
Open Music System (formerly Opcode MIDI System). A real-time MIDI operating system for Macintosh applications (and slated to be integrated into Windows 95). OMS allows communication between different MIDI programs and hardware, so that e.g. a sequencer could interface with a librarian program to display synthesizer patch names -- rather than just numbers -- in the sequencer's editing windows.

Operator
A term used in Yamaha's FM synthesizers to refer to the software equivalent of an oscillator, envelope generator, and envelope-controlled amplifier.

Oscillator
A synthesis module used to create a cyclical waveform. These monotimbral synthesizer may be able to play multiple notes of the one instrument sound simultaneously. In a digital synthesizer, an oscillator more typically plays back a complex waveform by reading the numbers in a wavetable.

Overdub
To record additional parts alongside (or merged with) previous tracks. Overdubbing enables "one-man band" productions, as multiple synchronized performances are recorded sequentially.

Overtone
A whole-number multiple of the fundamental frequency.