O'Brien was born in Portland, Oregon and adopted transracially (his birth parents are African-American and Finnish) when he was aged two by a family with two born-to and six adopted children.
He was a late bloomer athletically, not beginning discovered until he was at the University of Idaho. His adolescent and young adult years were difficult, including problems with alcohol and several days in jail for writing bad checks. He lost his athletic scholarship at college because of academic failure, but in 1987 he pulled himself together, enrolled in a junior college, and got his scholarship back. He has also had occasional problems with motivation, and has been disturbed by a spectacular failure in the pole vault at the 1992 Olympic trials.
He won the World Championship in the decathlon in 1993 and 1995 and the Olympic gold medal in 1996, holds the world record in the event, and is usually considered to be the greatest athlete in history.
He has attention deficit disorder (ADD).