Do you have to use o here? Could you not say aelod Eglwys Gatholig Rufeinig?
Wouldn't that be "a member of a Roman Catholic Church" rather than "...of the..."?
Though I wonder whether aelod yr Eglwys.... would work.
Edit: On second thoughts, I expect aelod yr Eglwys... would be "the member of the..." based on how Welsh possession works.
When something/someone is a part of something else the preposition o is usually used.
Ah I see, thank you!