On July 26, 2018, about 1855 central daylight time, an amateur-built Fisher Dakato Hawk airplane, N677TS, was substantially damaged while landing at Wittman Regional Airport (KOSH), Oshkosh, Wisconsin. The private pilot and passenger were not injured. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no Federal Aviation Administration (FAA) flight plan had been filed for the flight. The local flight originated about 1845. According to the pilot, he was landing on runway 33, at the ultralight airfield. After the airplane touched down the left main landing gear collapsed at the bottom end of the strut. The airplane ground looped, and the right wingtip struck the ground resulting in substantial damage to the wing rib. The pilot stated that the landing was smooth and gentle, not hard, and there was no side load on the landing gear at touchdown. He suspected that a previous landing may have been hard resulting in a crack in the landing gear. An examination of the left main landing gear revealed that the upper/outer strut tubing was broken, and the lower/inner strut tubing was bent. There was no evidence of fatigue. An examination of the airplane and related systems, conducted by a National Transportation Safety Board Investigator and FAA inspectors, revealed no anomalies that would have precluded normal operation.