Our biggest problem has been with raccoons (when we lived in a suburb in Lexington) and a neighbor's dogs. We got our own dogs and the neighbor's dogs stay away.
Opossums are pretty cool creatures, actually. They are marsupials which is pretty neat. http://opossum.org/ I don't think they eat chickens, just the eggs.
We have had opossums attack a few of our chickens. If they are hungry enough I guess. :0(.