Fr what sort of romanisation system uses an acute accent for that. It's much more akin to the latin flat accent. But in reality just leaving off an accent or using the u is more convenient for typing.
A number of European languages use ó for indicating the long vowel o, rather than stress: Czech, Slovak, Polish, Icelandic, Irish, ...
According to Wiktionary, ō is used as a long vowel in two Latvian languages, and in Swedish as a hand-written form of ö (not always a long vowel). Also in Silesian, a language in a region of what is now Poland.
