Not op, but I use this system which has instructions for installing to Linux, Windows, and macOS. [1]
It's described as "QWERTY for Western European languages" and supports quite a few characters with simple AltGr (right alt) key combos. Alt+o = ö Alt+a = ä
(American living in Europe, I haven't wanted to move from US QWERTY physical layout, but needed an extended character set in daily use)