2002 Explorer 4.0
I can understand leaking out of the top groove of thermostat housing where it separates from the rest of the housing, but the location where it's leaking is solid piece.
Am I wrong?
Would I need to replace just O-Ring? Or entire thermostat housing?

 

Being plastic, there could be a split allowing coolant to seep. It still looks like a gasket is at the seam located at the leaking point. I would say you need the whole thermostat housing or intake manifold if that area is intergrated with the manifold. It is hard to tell without a wide photo of the area..