William Patrick ("Parry") O'Brien (January 28 1932 – April 21 2007) was an American shot put champion. Born in Santa Monica, California, he competed in four consecutive Summer Olympics where he won two gold medals (1952, 1956) and one silver medal (1960). In his last Olympic competition (1964) he placed fourth. For this, he is inducted in the U.S. Olympic Hall of Fame.

In the early 1950s, O'Brien developed a new method for throwing the 16-pound shotput. The L.A. Times described it: When O'Brien began throwing the shot, the standard method was to rock back on one leg, swing the other in front for balance, hop forward and propel the iron ball forward. O'Brien instead began by facing the back of the circle. He then turned 180 degrees, using the spin to generate momentum and help him throw the shot greater distances.

Using this method he was able to break the world record 17 times, become the first person to throw the 16-pound shotput more than 60 feet, and win 116 consecutive competitions. This method became known as "O'Brien Style" or the "O'Brien Glide." He held the world record from 1953 to 1959 During his career he won 18 National Amateur Athletic Union championships, 17 in the shotput and one in the discus. He won nine consecutive national indoor shotput titles and won eight overall outdoors, including five in a row.

Parry was active in sports in high school, becoming an end on the Santa Monica High football team, and winning a state championship and a football scholarship to USC.

As a competitor, in addition to developing new techniques for the shotput, he also made motivational tapes for himself, and experimented with Yoga. Time magazine, in a cover story written during the week before the Melbourne Olympics, noted "None has been more successful than O'Brien in combining what he calls "M.A." (mental attitude) and "P.A." (physical aptitude)." He was the first man to retain his Olympic shot put title since Ralph Rose in 1904 and 1908. In the 1960 Olympics he won a silver.

In 1964, he was the flag bearer for the U.S. Olympic team at the Tokyo Games.

O'Brien was inducted into USA Track and Field's Hall of Fame in 1974, the U.S. Olympic Hall of Fame in 1984 and USC's Athletic Hall of Fame in 1994.

O'Brien died in the 50-meter pool at the Santa Clarita Aquatics club during the Southern Pacific Masters Association regional swimming competition. He was 75 years old.