Editorial decision.
There are four ways of writing the name of the
mathematician and the things named after him: Mobius, Moebius, Möbius, and Möbius.
Unfortunately the E2 search and hardlink mechanism does not recognize o and ö as the same letter, and of course oe will not match either. And HTML entities like ö should never be used in node titles. So I have decided the best compromise is to use plain O, and move all relevant write-ups across to the plain-O title.
If in due course the search mechanism is improved, it will be possible to give Herr Möbius and all his works the umlaut they deserve.
-- Gritchka
Of course this does not apply to nodes in which some other spelling is part of a different proper name, as in the graphic artist noded above.