". . . According to AN ENCYCLOPEDIA OF OXFORD PUBS, INNS AND TAVERNS by Derek HOney (Oakwood Press, 1998) . . . a Green Dragon stood in St. Aldate's, by Christ Church, until 1926 when it and two other pubs were demolished to make way for the War Memorial Garden. This is also close to Pembroke College, to which Tolkien was attached beginning in 1925, so it's possible that he had a pint or two in Oxford's Green Dragon in his day . . ."
I'm not surprised that Tolkien might have patronized a real Green Dragon, but had I known that there was one that close to his own college at Pembroke that first year after he returned from Leeds, I would certainly have included the fact in MR. BAGGINS. A nice little discovery on David's part.