O'Brien narrowly missed a medal in the 4 ×  100 m medley relay. Along with Michael Wenden, Robert Cusack and Karl Byrom, the Australian quartet won their heat and entered the final as the equal fifth fastest qualifier. In the final, O'Brien swam his leg in 1 min 8.6 s, which was only the fifth fastest breaststroke leg. Australia were fourth at the end of each leg, except O'Brien's, when they were third. Australia eventually missed out on the bronze by 0.1 s to the Soviet Union. O'Brien admitted that his training had been insufficient for Olympic standards, noting that "I needed to put on another thousand kilometres in training". O'Brien also rued the absence of Talbot to motivate him to work, and had a further accident at the Olympic Village when his fingers were slammed by a closing window. Under competition regulations, he was not allowed to bind his hand during competition.