ő

The idea of using a diaeresis (¨) to distinguish between front vowels and back vowels originates from German; see diaeresis for the full scoop if interested. The sound ö (or ø in IPA for any linguists out there) does not occur in English, but it is similar to the initial vowel of "earth"; if you alternate between saying "o" and "earth", you can feel how the place the sound is coming from changes. Well, that's the difference between front and back, and the Germans used ö for the frontal version.

Still with me? Now, German (and English) orthography doesn't really distinguish between short and long vowels, as vowel length is not very important in Germanic languages. However, both Finnish and Hungarian imported their orthography wholesale from German (Finnish via Swedish, Hungarian thanks to the empire of Austria-Hungary), and since in the Finno-Ugrian language family vowel length is crucial, some way of marking this had to be developed. The Finns took one route by decreeing that long vowels would be doubled: "a" is short, "aa" is long, and likewise "ö" is a short Ö, "öö" a long one. No problem!

But Hungarian went for a different tack: they decided to use an acute accent to tag long vowels, so that "a" is short and "á" is long. Fine enough for everything else, but how to represent the long "ö" sound? Well, the following little table makes the answer obvious:

       short  long
back     o      ó
front    ö     ???