FWIW I think 言う is a different phenomenon entirely, because おう is pronounced as two vowels when it has grammatical meaning (in this case, as the verb ending), or between different words/morphemes. But my (non-native) understanding was that for nouns and such, or within the main morpheme of a verb (e.g. 葬る), “ou” is (usually) indistinguishable from “oo”.