According to https://github.com/cocur/slugify/blob/master/Resources/rules/austrian.json#L3 and https://github.com/cocur/slugify/blob/master/Resources/rules/german.json#L3 "Österreich" will be slugified to "OEsterreich", but it should be "Oesterreich". However, when it's all uppercase "OESTERREICH" is correct (not "OeSTERREICH"). So the perfect (?) solution probably would be to check if the next character is uppercase too, then go for E or e.
According to https://github.com/cocur/slugify/blob/master/Resources/rules/austrian.json#L3 and https://github.com/cocur/slugify/blob/master/Resources/rules/german.json#L3 "Österreich" will be slugified to "OEsterreich", but it should be "Oesterreich". However, when it's all uppercase "OESTERREICH" is correct (not "OeSTERREICH"). So the perfect (?) solution probably would be to check if the next character is uppercase too, then go for
Eore.