Onsham started his career as the club professional for
Royal Bangkok Sports Club in
Bangkok, Thailand. He participated in the World Cup in 1969. Onsham was the first man from Thailand to play in the Masters, playing in the
1970 and
1971 events. He missed the cut both years. He is often referred to as the first non-white person to play at the Masters; this is false. The Japanese golfers
Torakichi Nakamura and
Koichi Ono played in the
1958 Masters Tournament well before Onsham. Also several other players from Japan, Taiwan, and the Philippines played in the event during the 1960s. Onsham had much success in 1972 on the international circuit. Early in the year at the
Malaysian Open, he finished one behind Japan's
Takashi Murakami, tying
Walter Godfrey and American
Marty Bohen for second. On 23 October 1972, Onsham finished runner-up to American
Bob Murphy at the 1972
Wills Masters, an event on the
Australian Tour held outside of Sydney. Two weeks later he posted another runner-up in Australia at the
Dunlop International, four shots behind
Tony Jacklin, finishing tied for second with
David Graham. That year he also placed third at the
Japan Open Golf Championship. Two years later, Onsham won an event, the
Malaysian Dunlop Masters, defeating Australia's
Ted Ball and Malaysia's Bobby Lim by two shots. Onsham's career began to wind down in the 1980s, however, he did have some success that decade. He finished runner-up at the 1981
Indonesia Open. A young
Payne Stewart made a 20-foot birdie putt to defeat him and Taiwanese golfers Hsu Chi-San and
Chen Tze-chung on the first playoff hole. As of 1988, he was a golf instructor. Onsham also represented Thailand multiple times in the World Cup. As a senior, he played on the
European Senior Tour and Japan Seniors Tour from 1998 to 2000. ==Professional wins==