Is there a difference in pronunciation between おお and おう?
Hello. I have been learning Japanese for about a year now but recently I've come across words that don't use "ou" to lengthen the "o" and instead use "oo" (like tooi or ooki). Is there any difference between the pronunciation, besides in verbs ending with "ou" (like how "omou" does not have a lengthened "o" at the end but rather a separate "o" and "u" sound)? How do I know when to use each and is it okay to use them interchangeably? Sorry if this sounded confusing. I tend to pronounce words like "tooi" like "to-oi" with a very short pause between the two "o" sounds, but I have a feeling that's very weird if that makes any sense whatsoever. Anyways, thank you in advance!
There is no difference in pronunciation between the two, but using them in different contexts might not be considered correct.
You can't spell とうきょう as とおきょお. It would not be considered correct spelling. It should be pronounced the same, but since おお occurs so rarely, people might be tempted to put pauses between "o" sounds (which I belive is how you pronounce 女王 (じょおう, meaning "queen"), kinda like jo'ō). Likewise, spelling とおい or おおきい as とうい or おうきい would also be incorrect, and since it's not the accepted spelling, people might be tempted to read it with a as "o pause u," kinda like "to'ui" or "o'ukii."
おう is the standard spelling of the long "o" sound, and the only reason おお is still used in a few words is historical reasons.
Usually a long o sound would be written as o- (オー) in katakana and ou (おう) in hiragana. There may be a subtle difference in pronunciation but I've never really paid it too much mind and no one has complained yet. I suppose it's possible they're humoring me.
When there's an actual oo noise written out in hiragana, there's usually a reason. I read (in The Phonology of Japanese by Laurence Labrune) that words such as ookii (big) and koori (ice) used to have a oho sound, like ohokii or kohori. The ho noise became just a long o over time, but the spelling still retains a bit of that history. Pretty much, you can just memorize those specific words when they come up, because they're unusual.
Okay, I just looked it up again and The Phonology of Japanese says that the second vowel in a double vowel situation can be different than a long vowel because it can carry the accent (which I'm reading as emphasis in this context), whereas the second half of a long vowel cannot. So for instance you could put a little oomph on the second u in mizuumi (lake) to make it sound more like a compound word than a long u. The double-o related examples given are tookereba (if it is far) and ooi (many).