Otto, Frei (frĪ ôˈtō) [key], 1925–, German architect. Most notable for his tensile and pneumatic structures, Otto is among the first major architects to experiment with lightweight design. He is a noted designer of exhibition tents and sports arenas. Otto has also experimented with pneumatic membranes stabilized and maintained by means of air and gas pressure. Structures built with these membranes are used for oil-storage tanks, grain silos, and greenhouses.