Led off by O'Callaghan and brought home by Titmus, with Lani Pallister and Brianna Throssell in the middle legs, they touched in an Olympic-record 7min 38.08sec.
The Katie Ledecky-led United States (7:40.86) took silver and defending champions China (7:42.34) the bronze.
It avenged Australia's failure to win gold in Tokyo when China set a new world record to stun both them and the Americans.
Australia reclaimed the world record at the 2023 world championships and were heavy favourites in Paris.
Their time at La Defense Arena was second only to their own world best.
They got a flying start from O'Callaghan, who clocked 1:53.52 to give them the lead, but it was Titmus who sealed the deal, coming home in a sizzling 1:52.95 to leave the USA and China in her wake.
