For alphabets with dots one has
ö=\"o, but I would like to introduce another horizontal line just above the two dots. The command
\=\"o doesn't yield the sought result.
Well, superimposing accents is not always easy; even the advice of
I've put next the result of superimposing the accent. Act on
@egreg's answer wasn't working for me on XeLaTeX, but I found something that works OK.