The Outerbridge-Kashiwagi (O-K) procedure is one of popular procedures for the treatment of osteoarthritis of the elbow. Although reliable outcome has been reported in the literature, intraoperative and postoperative complications may occur. Acute postoperative neurologic complications are rarely reported in the literature. We report a case of acute complete ulnar neuropathy following O-K procedure in the elbow with longstanding flexion loss. Prophylactic ulnar nerve decompression during the O-K procedure should be considered in the elbows with osteoarthritis and prolonged severe flexion contracture.