In 1967, O'Brien skipped the Australian Championships because he had no sponsorship and ran out of money, forcing him to seek full-time work. In 1968, despite Graham Edwards winning the National 200 m breaststroke title, the Australian Swimming Union persuaded an overweight O'Brien to make a comeback in 1968 on the grounds that Australia did not have a quality breaststroker for the Olympics. Undergoing a crash diet and fitness program, O'Brien lost 12.7 kg in twelve weeks of intense training. O'Brien was unable to reclaim either of his individual Australian titles, but New South Wales again won the medley relay. Nevertheless, he was selected for his second Olympics.