Which O ring do you mean please? 41 or 2 in the diag above are on the brand new adapter.
There is a broad gasket on the inside, but I’m not getting any water on the underside of the water tank itself.
I’ve got some food grade lube - is that suitable? I suspect from the reddish colour that the adapter gaskets themselves might be silicon.
From your response this ring (visible below) might be the culprit, despite being brand new. The ring at the top sealing the hole in the tank is tiny, fitting inside the metal adapter.