Ivor Darreg (May 5 1917 - 1994) was a leading proponent of and composer of microtonal or "xenharmonic" music.

Darreg was born Kenneth Vincent Gerard O'Hara in Portland, Oregon. His father John was editor of a weekly Catholic newspaper and his mother was an artist. He dropped out of school as a teenager, but taught himself at least ten languages and had a basic understanding of all the sciences. His real love was music and electronics. Because of his choice of music, his father cast him out, and he and his mother set out on their own with little help from anyone. At that point he took on the name "Ivor," which means "man with bow" (from his cello-playing talents) and "Drareg" (the retrograde of "Gerard"), which he soon changed to "Darreg".

Darreg lived for much of his adult life in Los Angeles, California, then spent his final 9 years in San Diego. He coined the term "xenharmonic", designed and built many original microtonal musical instruments, and wrote voluminous amounts of material about various musical tunings. Perhaps his most important contribution to music theory was his idea that different tunings exhibit different "moods".