Oldfield was born in Elgin, Illinois, and began his career at Middle Tennessee State University where he won the Ohio Valley Conference championship three times. The University recognized his achievements by inducting him into their athletic Hall of Fame in 2000.
After holding several jobs, Oldfield set his sights on achieving stardom in the shot put as an Olympian. In 1972, he made the US Olympic team, but finished in 6th place. He bounced back less than a year later by setting his first world record, with a throw of 21.60 m (70'10½"). However, this record was not official due to his affiliation with professional track & field.
In 1975, his throw of 22.86 m (75') set another unofficial world record. Though unofficial, Oldfield's accomplishment did not go unnoticed. After setting this mark, he had earned a cover spot on Sports Illustrated, and also made an appearance in a 1975 issue of Playgirl. In his Sports Illustrated interview, he confidently asserted that he expected to be throwing over 80' before 1980. In 1984, at age 38, he finally set an official record with a throw of 22.19 m (72'9") to set a new American mark. When asked how he was able to do it by a commentator at the event he responded "I had a 'throw-gasm.'"
But Oldfield was perhaps at least as well known for his unconventional persona and on-field antics as he was for his athletic performance. Unusually for track athletes at the time, he wore his hair long in a style he dubbed the "Oldfield Mop" and occasionally sported a beard. Oldfield would sometimes smoke cigarettes in between throws at competitions to show that he could beat anyone, even while smoking. He was known for wearing flamboyant outfits, including tie-dyed shirts and Speedo-style shorts. These stunts served not only to raise Oldfield's profile, but frequently unnerved his opponents. At the 1972 US Olympic Trials, an opponent was quoted as saying "I will retire the day that I lose to someone like Brian Oldfield." Not surprising for the man who said in the Sports Illustrated article about him: "When God created man, he wanted him to look like me." (This quote was in the September 1, 1975 Sports Illustrated article about him.)
Oldfield competed in the World's Strongest Man contest in 1978, finishing seventh in a field of ten competitors. He also competed in Scottish Highland Games in the 1970s. Utilizing his experience in the shot put, he set many field records in the Stone put. His career-best throw of 63'2" in the light stone, accomplished at Braemar, Scotland, in 1973, was a world record until 2013.
Oldfield also starred in the 1989 film Savage Instinct, later renamed They Call Me Macho Woman! as Mongo, the crazed drug lord. In the film, Oldfield wears a special spiked headgear that his character uses to head-butt people to death. The movie was unsuccessful.
Near the end of his life, injuries from his time in competition reduced the athlete to walking with a cane and using a wheelchair.
Oldfield died on March 26, 2017, at his home in Elgin, aged 71.