Long "O" Romaji

Hi I’m pretty new to Japanese and had a question about Romaji. I noticed when a word has a long “o” sound it is extended usually with a う. However, there are some cases where the “o” sound is extended with a お like the “too” sound in お父さん and I always mess it up in my reviews. Can anyone explain the difference? It seems like the same to me.
Thanks!

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Welcome to the site!

When we’re just talking about the lengthening, there is no difference in pronunciation.

とう and とお should sound the same as long as it is meant to be a long “oh” sound.

(The difference would be when the following kana is not part of the same sound, just coincidentally following it, such as in the verb う. In the case of 問う, you would say と+う as distinct sounds, not a long とー.)

Anyway. getting back to lengthened ones, the difference is one of history. The long vowels lengthened with う come ancient Chinese, and the long vowels lengthened with お come from ancient Japanese. The spellings used to be a lot weirder, but they were systematically revised in the last 100 years.

It’s pretty much just a matter of getting used to them and memorizing the spellings.

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You just have to remember case by case. The staggering majority of お sounds are lengthened by う and え by い, but there are exceptions where お is lengthened by お and え is lengthened by え, so those you have to memorise.

But your question was about rōmaji and you’re not really asking about rōmaji, but about typing Japanese in IME.

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As with most things, it will become second nature after you get used to it.

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Others have already explained the pronounciation by now. Fortunately, these are rare exceptions and there are relatively few of them. You have to learn that on a case by case basis.