Onat was born in Istanbul in 1929 and moved to Izmir in 1930 with his family. He started playing chess at İzmir Atatürk High School, graduated from the Faculty of Pharmacy of Istanbul University. Onat, who met strong players while he was a student in Istanbul, took a break from chess when he returned to Izmir. As a chess player, he took part in the 17th, 18th, 25th, 26th and 27st Chess Olympiad Turkish National Team. He won the 1974, 1975 and 1982 Turkish Chess Championships.