From Star Trek: Theurgy Wiki
The USS Defiant began its service in when it officially launched in 2370, on stardate 47538.5 from the Antares Shipyards and served until its destruction during the Second Battle of Chin'toka in mid-2375. Originally considered a failed prototype, the escort was placed in the service of the base commandant of Deep Space 9 and performed so far above expectations that the class was put into production by the time Dominion War started.
Sinead O'Riley served aboard the Defiant in 2373.
After the Second Battle of Chin'toka the USS Sao Paulo was assigned to DS9 to replace the Defiant after its destruction. DS9’s command staff was granted special dispensation by the Chief of Starfleet Operations to rename the Sao Paulo to "Defiant" in honor of this "tough little ship." (DS9: "The Dogs of War")