I think it's not the right pump in the picture and the oring is on the propeller side right in the block. Looking for a pic still.
Edit: Wait, I see you changed the picture. From the looks of it the oring should be inside the housing you posted like you can almost see it through the hole in the pic. I bought 3 pumps through the years and they came without this housing, but pretty sure they bolt right to the block.
This is what a 2.5 water pump looks like:
Edit2: Sorry, it seems you have the right picture and the pump indeed attaches to a housing. It's between this housing and the pump where the oring groove is.
[This message has been edited by cebix (edited 10-13-2020).]