| Oboe | Euterpea.Music, Euterpea |
| Ocarina | Euterpea.Music, Euterpea |
| Octave | Euterpea.Music, Euterpea |
| offset | Euterpea.Music, Euterpea |
| OpenCuica | Euterpea.Music, Euterpea |
| opened | Euterpea.IO.MIDI.MidiIO, Euterpea.IO.MIDI, Euterpea |
| OpenHiConga | Euterpea.Music, Euterpea |
| OpenHiHat | Euterpea.Music, Euterpea |
| OpenTriangle | Euterpea.Music, Euterpea |
| OrchestraHit | Euterpea.Music, Euterpea |
| OrchestralHarp | Euterpea.Music, Euterpea |
| Orn | Euterpea.Music, Euterpea |
| Ornament | Euterpea.Music, Euterpea |
| osc | Euterpea.IO.Audio.BasicSigFuns, Euterpea.IO.Audio, Euterpea |
| oscDur | Euterpea.IO.Audio.BasicSigFuns, Euterpea.IO.Audio, Euterpea |
| oscDurI | Euterpea.IO.Audio.BasicSigFuns, Euterpea.IO.Audio, Euterpea |
| oscFixed | Euterpea.IO.Audio.BasicSigFuns, Euterpea.IO.Audio, Euterpea |
| oscI | Euterpea.IO.Audio.BasicSigFuns, Euterpea.IO.Audio, Euterpea |
| oscil | Euterpea.IO.Audio.CSound |
| oscil1 | Euterpea.IO.Audio.CSound |
| oscil1i | Euterpea.IO.Audio.CSound |
| oscili | Euterpea.IO.Audio.CSound |
| oscils | Euterpea.IO.Audio.CSound |
| oscPartials | Euterpea.IO.Audio.BasicSigFuns, Euterpea.IO.Audio, Euterpea |
| outA | Euterpea.IO.Audio.Basics, Euterpea.IO.Audio, Euterpea |
| outFile | Euterpea.IO.Audio.IO, Euterpea.IO.Audio, Euterpea |
| outFileNorm | Euterpea.IO.Audio.IO, Euterpea.IO.Audio, Euterpea |
| output | Euterpea.IO.MIDI.MidiIO, Euterpea.IO.MIDI, Euterpea |
| OutputDeviceID | Euterpea.IO.MIDI.MidiIO, Euterpea.IO.MIDI, Euterpea |
| outputMidi | Euterpea.IO.MIDI.MidiIO |
| OverdrivenGuitar | Euterpea.Music, Euterpea |