Arriving in Tokyo, O'Brien was nominated in the 200 m event and the medley relay; the 100 m event was yet to be included in the Olympic program. The favourites for the 200 m breaststroke were Chet Jastremski of the United States—the world record holder—and Georgy Prokopenko of the Soviet Union. Gathercole had modelled O'Brien's technique on that of Jastremski, attempting to refine and smoothen it. Years after O'Brien retired, Harry Gallagher said that "Ian O'Brien has an almost faultless style and is a great example for Australian youngsters to copy". O'Brien was known for the strength that his torso generated, and his powerful kicks; sports science experiments showed that his vertical jump was especially strong. O'Brien was also known for his efficient start off the block. He often gained a lead of approximately a metre from his dive and underwater glide at the start, and was able to complete 50 m in 31.0 s.