River to River Festival. Original Music Workshop (OMW), an exciting Williamsburg arts incubator masterminded by composer and pan-disciplinary artist Paola Prestini, won't open its doors until 2014. But that hasn't prevented Prestini & Co. from causing a stir; here, OMW hosts Ex-Situ, a series of site-specific events. Mesmerizing Norwegian singer-soundscaper Maja S.K. Ratkje gets things started tonight; "The Violin," a collaboration among violinists Cornelius Dufallo and Amy Kauffman, composer Anna Clyne and visual artist Josh Dorman, follows on July 9; and the series ends with a performance by pianist-composer Hauschka, percussionist Samuli Kosminen and cellist Jeffrey Zeigler on July 11. Reservations are required for "The Violin," so make one at tfaforms.com/289781.