The O/100 was an unequal-span three-bay biplane, with the overhanging part of the upper wing braced by kingposts, a rectangular section fuselage and a biplane tail with twin balanced rudders, between the horizontal surfaces. Balanced ailerons were fitted to the upper wing only and extended beyond the wing trailing edge and wing tips. The engines drove four-bladed propellers, rotating in opposite directions to cancel the torque, and were enclosed in armoured nacelles mounted between the wings on tubular steel struts. The nacelles had a long tapered fairing to reduce drag. To clear the wing rigging wires when the wings were folded, the rear portions of the fairings were hinged to fold inward. The structure of the fuselage and flying surfaces was primarily spruce, with the spars routed into I-beams to reduce weight.