Dan O'Brien was born on July 18, 1966, in Portland, Oregon, and lived in an orphanage until he was adopted into a large, multiethnic family in 1968. He excelled in football and basketball during high school and won numerous state track and field titles. Although he earned a scholarship to the University of Idaho, academic ineligibility kept him sidelined for several seasons. After some time at a community college in Spokane, Washington, a more mature O'Brien returned to Idaho and was expected to win the national collegiate title in the decathlon in 1989, but an injury prevented him from competing.
O'Brien placed second in his first international meet, the 1990 Goodwill Games. He won the decathlon at the national track and field championships for the first time in 1991 and went on to capture the world title. In 1993 he successfully defended his world decathlon title and became the world indoor heptathlon champion by scoring a world-record 6476 points. He continued with decathlon victories at the 1994 Goodwill Games and the 1995 world championships.
Despite these accomplishments, Olympic success eluded O'Brien. He led after two events at the 1988 Olympic trials, but a long-jump injury forced him to withdraw. In 1992, after becoming a celebrity by starring with fellow U.S. decathlete Dave Johnson in a series of popular pre-Olympic advertisements, O'Brien never made it to the competition in Barcelona, Spain; he missed all three pole vault attempts at the Olympic trials and finished 11th overall. He managed to redeem himself somewhat at a meet later in the year at Talence, France, by setting a world record of 8891 points, outscoring Olympic champion Robert Zmelik by more than 500 points.
Between the 1992 and 1996 Olympics O'Brien had to overcome various obstacles, including physical injuries, a drinking problem, and mental barriers. A sports psychologist helped him improve his attitude toward the 1500-meter race, the last event of the decathlon and the one he disliked the most. Diagnosed with attention deficit disorder, he learned techniques to improve his focus. The efforts paid off—O'Brien scored 8824 points at the Atlanta games to win the gold medal.