He won both breaststroke events at the 1963 Australian Championships, repeating the feat for the next three years. In 1964, O'Brien went to the Tokyo Olympics and came from third at the 150 m mark to win the gold medal. He added a bronze in the medley relay. O'Brien successfully defended both his breaststroke titles at the 1966 British Empire and Commonwealth Games in Kingston, Jamaica before retiring to support his family. Swimming officials persuaded him to make a comeback for the 1968 Summer Olympics in Mexico City, as Australia did not have a breaststroker, and after a crash diet, he finished sixth in the 100 m event but failed to reach the final in the 200 m event. He then retired and went into the television industry.