The 32-year-old Scot birdied the 18th hole to shoot a 5-under 67 and force the playoff at 18-under 273. Forsyth then birdied the par-4 18th again in the playoff for his first win since the 2002 Malaysian Open.
Forsyth trailed Otto (72) by five strokes heading into Sundays final round.
Gary Clark (70) was third, four strokes behind, and Sven Struver (69) was fourth at 278.
Frances Michel Lorenzo-Vera and Fredrik Widmark of Sweden trailed by seven strokes at 208.