I suppose you can't go wrong with what your own passport says
On second thought ...
* disallowed: Ä↛A , Ö↛O , Ü↛U (as are: Å↛A , Ø↛O)
... I have a Turkish friend for whom it is Ö→O, not OE. This calls into
question the general applicability of these rules. A few years ago he
also told me that it's nice that Germany has recommended replacement
rules because Turkey doesn't. The linked-to document is dated 2006, but
he told me this after 2006. His knowledge might have been out of date
(maybe Turkey now does Ö→OE), but in the light of this the extent to
which these rules reflect popular usage remains very much unclear, and
we all seem to be agreeing that it'd be unlikely if practice were
uniform anyways.
Stephan
