> that the "-o/-oe" place names in Scotland and Cornwall ... are ultimately holdovers from British

Maybe in Cornwall. In Scotland not in every case. Glasgow is certainly British (= "green hollow") , but Lesmahagow is Gaelic (lios mo-fhegu = "enclosure of my (Saint) Fechin" - at least according to Watson's identification of the saint's name). In eastern Scotland, where Pictish gave way to Gaelic and then Gaelic gave way to Scots later in the Middle Ages, there are a number of placenames where Gaelic -ach became -o (Pitsligo, Stracathro, Balmerino) - also Balerno in the SE, where there are a few Anglicized Gaelic placenames among others of British and Old English origin. See http://www.arts.gla.ac.uk/STELLA/STARN/lang/GAELIC/evident.htm for more on these.

Beyond that I'd be getting beyond my depth too. Better get back to the topic I guess...