(covers information from several alternate timelines)
Overseer was a supervisory title given to someone in charge of monitoring the work of others.
In 2370, Major Kira Nerys and Chief Miles O'Brien infiltrated the Hutet labor camp on Cardassia IV by bribing a Cardassian overseer with an offer to use Kira's "services" for just two strips of latinum. When the overseer lowered the force field protecting the camp, he was attacked and subdued by the two. (DS9: "The Homecoming")
In the mirror universe, the Klingon-Cardassian Alliance had overseers working in the ore processing centers on Terok Nor during the 2370s. In 2371, Garak gave the overseers beneath him permission to execute any worker not meeting their quota after overall productivity fell by fifteen percent. When this failed to work, Superintendent Kira noted that it only served to please the overseers and elected to perform "random and unprovoked" executions to "motivate" the workforce. The following year, "Smiley" O'Brien lured Captain Benjamin Sisko into the mirror universe by kidnapping his son in order to convince him to build a Defiant-class starship to assist the Terran Resistance Forces. After Sisko initially refused, O'Brien reminded him that unless he helped, the Alliance would either kill him and his son or force them to work in the ore processing center "under the tender mercies of an Alliance overseer." (DS9: "Through the Looking Glass", "Shattered Mirror")