While technically the word overseer means nothing more than supervisor, it has come to be associated with slavery to such an extent that it can hardly be used without bringing up that association. A passage in the Wikipedia http://en.wikipedia.org/wiki/Slavery article provides an example where "overseer" wouldn't ordinarily be replaced with "supervisor":
The overseers in many of these areas literally worked the slaves to death.
This connotation has become closely attached to the meaning of overseer and needs be addressed in the definition. Referring to one's supervisor as an overseer is not complimentary; it's roughly equivalent to calling one's workplace "the salt mines".