At the 2004 Summer Olympics in Athens, Ong shortened his program, swimming only in two individual events. He cleared FINA B-standard entry times of 23.49 (50 m freestyle) from the Malaysian Open Championships in Kuala Lumpur, and 51.57 (100 m freestyle) from the Southeast Asian Games in Hanoi, Vietnam. In the 100 m freestyle, Ong challenged seven other swimmers in heat four, including fellow two-time Olympians Alexandros Aresti of Cyprus and George Gleason of the Virgin Islands. He edged out Bulgaria's Raichin Antonov to take a seventh spot and fiftieth overall by 0.29 of a second, outside his record time of 52.04. In his second event, 50 m freestyle, Ong matched his forty-sixth place tie with SEA Games champion Arwut Chinnapasaen on the morning's preliminaries. Swimming in heat five, he managed to pull off a fifth-place effort in 23.52, just 0.03 of a second off his entry time.