(covers information from several alternate timelines)
The ore processing center was an area on Terok Nor which refined uridium ore mined from Bajor into its pure form for industrial use. Slave labor was used to move and process the ore. Ore processsing was a process carried out in temperatures as high as 55 degrees Celsius. (DS9: "Babel", "Wrongs Darker Than Death or Night"). A worker revolt in ore processing would trigger the Counter-insurgency program (DS9: "Civil Defense").
After the Cardassians left Bajor and Terok Nor in 2369, the ore processing center remained unused. In 2371, Miles O'Brien wanted to convert Deep Space 9's ore processing centers into Deuterium refineries. (DS9: "Civil Defense")
In the mirror universe, Odo was in charge of the center aboard Terok Nor until he was killed by Julian Bashir in 2370. He was replaced by a Cardassian overseer after his death. Terran slave labor was used instead of Bajoran slaves, as Bajorans were members of the Klingon-Cardassian Alliance (DS9: "Crossover ", "Shattered Mirror")