O-Ring? I did take off the housing from the engine block (the bit with the tower for the thermostat), as well as separating the water pump from the housing itself. My WP kit came with an o-ring, but I did not see any place to use it. There was one smaller o-ring on the smaller tube that ran through between the heads, that connects to the back side of the housing.

The WP kit came with 2 gaskets, one that went in between the WP housing and the engine block, and one that went between WP and the WP housing...

I did not separate the thermostat housing from the WP housing