Interesting. Still, I just wanted to point out that it’s only a general rule. Far more common words come to mind, like 今日(きょう)or おとうさん.
I think this is generally true, but I’ve never really looked into how Japanese people feel about it. Or perhaps what I’d be more interested to know is how Japanese people learn to pronounce these sounds. Are they explicitly taught as children that they’re the same? At the same time though, I honestly only do this ‘O-O’ thing for the long O sound, because (almost) none of the other long vowel sounds carries a similar risk of confusion.
I think this is true too, but that doesn’t mean there aren’t exceptions or cases of the two being interchangeable. What I mean by this is that the two sorts of pronunciation might both be acceptable, and it’s a matter of how much the speaker wants to separate the two sounds. For example, the instructor for the Japanese Pronunciation for Communication course on edX by Waseda University pronounces 一方 with quite a sharp drop in pitch in one of her lessons, saying ぽ and う (or お – it might have been an extended O sound, of course) almost as though they were two sounds. There are two ways to drop pitch, after all: smoothly or abruptly. She pronounced it with less emphasis in another lesson, and the second mora, whatever it was, definitely wasn’t as noticeable.
At the end of the day, perhaps I’m just a little too attached to my ‘trick’ to let go of it, but a big part of why I do it is because it works. I see so many people asking the very question that started this thread (this isn’t my first time responding to this issue), and I’m just glad I never need to ask it myself. Also, like I said, I’m pretty confident that what I do sounds the same, since I don’t allow myself any glottal stops between the two vowels.
Final justification: it’s kinda like what Japanese singers do when they pronounce 嗚呼(ああ)in the middle of a song – they use the same amount of force for the second vowel and change the pitch (e.g. あたしが隣にいる, the final line before the first chorus). Some words like 多い can be pronounced with the accent on the second syllable (LHL), and I think it’s pretty difficult/unnatural to attempt a smooth pitch transition, especially for such a short word where it’s pretty hard to do a ‘legato’ pitch variation. I’m trying to pronounce it both ways right now, and if I want something perfectly smooth, I need to slow down. Forcefully putting breath into the second syllable to get the right pitch is almost reflexive (I’m not talking about using a ‘stress accent’ here; it’s natural to inject a little more breath into your tone when increasing your pitch rapidly – almost all of us do it when singing) and tends to make it sound like a separate vowel even if I do my utmost to avoid any glottal stops. Both methods of pronunciation are practically equivalent as long as the pitch difference is large enough, and I think most Japanese pitch variations are about a third apart. That’s a fair jump if you’re speaking fast and need to raise the pitch of (i.e. accent) the second syllable, and difficult to do perfectly smoothly. That’s also probably why pitch drops tend to sound like separate vowels to many people: it’s hard to make them sound like exactly one smooth sound sliding down a third.