Dunno about OMN, but Lilypond seems to have quarter tone support: <a href="http:&#x2F;&#x2F;www.lilypond.org&#x2F;doc&#x2F;v2.19&#x2F;Documentation&#x2F;notation-big-page.html#accidentals" rel="nofollow">http:&#x2F;&#x2F;www.lilypond.org&#x2F;doc&#x2F;v2.19&#x2F;Documentation&#x2F;notation-big...</a> (third staff down)