Either; it's also possible (in older books) to find it with a diæresis over the [STRIKE]first[/STRIKE] second*
o, but I don't know how to type that. It looks a bit like an umlaut, but it's not the same.
Until recently, I was keeping a collection of editions of the same dictionary, so I could trace this sort of evolution. But there's a limit to the number of old books that MrsK will tolerate. ;-)
b
*Coolfootluke kindly corrected me in a PM, thinking to spare my blushes. But I can take it ;-)