I am learning hiragana and I've encountered a minor problem. The thing is that my book did mention how to deal with a long "O": just by adding another "O" or "U". However it doesn't explain in which cases I should use which.

As you can see I've made a mistake trying to put "O" in 7th case. In which cases of double "O" should I use "O" or "U"?

My problem

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    I think you should worry more about the size of your よ here. You've written a big よ when it should have been a small ょ. This matters a lot. Your question will become irrelevant when you stop using romaji; which you should do as soon as possible. – user3856370 Feb 15 '17 at 17:48
  • @user3856370 That doesn't make any sense. You still need to know that 王{おう} contains a long vowel and 追{お}う does not, and the transcription (ō and ou) is just a way of indicating that difference in writing. You can transcribe the pronunciation in katakana if you prefer (オー and オウ), but you need to know the difference whether you're using romanization or not. Learning to use the Japanese writing system is probably a good idea, but it won't solve the OP's problem for them. – snailboat Feb 15 '17 at 18:02
  • @snailplane I agree entirely. I wasn't suggesting that you don't need to know the difference. Merely that what ō means doesn't matter because you won't ever see ō again. – user3856370 Feb 15 '17 at 18:32
  • You remember it. There are not many words with お. Most of them use う now. – fefe Mar 23 '17 at 1:26
  • Related question: japanese.stackexchange.com/q/3939/1628 – Earthliŋ Nov 12 '17 at 22:12

Really, all I can say is 'it depends on the word'. Generally on'yomi (Chinese-derived) readings use おう, while kun'yomi (native Japanese) readings use おお, but there may be exceptions. A note: if う is a verb ending, おう will not be pronounced おお but as お and う separately, as in 追う and 思う. A lot of what I've said also applies to えい and ええ.

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    More specifically, they are pronounced separately if there is a morpheme barrier between them. – Kurausukun Feb 15 '17 at 22:12
  • What you say is only valid for the infinitive form of the verb, 呼ぼう is pronunced with a long 'o', not with a 'u'. – jmd Mar 19 '18 at 13:13

As @Nothing at all notes, this depends on the word.

On'yomi always use -OU for long O sound and kun'yomi almost always -OO.1

However, the real problem here is that you are being asked to reconstruct hiragana from Hepburn romanization. In general this is impossible, because Hepburn romanization conflates certain hiragana spellings. (There are romanization systems that don't.)

The fact that there are many homophones is probably one of the reasons that kanji still exist in Japanese. Without kanji and only hiragana, we would have that 糖衣、東夷、当為、等位、… would all be とうい.

Hepburn romanization goes even further, and also conflates とうい (e.g. 糖衣、東夷、当為、等位、…) and とおい (e.g. 遠い) to tōi.

So, you would need a lot of information to know that there is no word しょおゆ, so that shōyu would have to be しょうゆ. However, there would be no way for you to know (without context) whether tōi should be とうい or とおい.

The only way to solve this type of question is to know the hiragana spelling of the words that come up. (Well, it suffices to know the words in a romanization system that does not conflate hiragana spellings.)

All of that said, a long O (in Hepburn ) is more likely to be -OU, so the most economical approach would be to default to -OU for and learn words with -OO such as とおい、とおる、おおい、おおきい、… as exceptions.

1 For kun'yomi there are rare exceptions that arise from a sound shift as in 妹【いもうと】 from いも+ひと which also occurs as a long O sound in 弟【おとうと】, 素人【しろうと】, 客人【まろうど】, 若人【わこうど】, 蔵人【くろうど】. Across "word boundaries" Hepburn romanization also uses -ou-; the above exceptions used to be word boundaries, but now they are not anymore, so they have a long O sound, romanized as -ō- in Hepburn.)

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    Now that I've thought about it, I can think of a few 訓読み おうs. You've got 妹{いもうと}, 弟{おとうと}, and 若人{わこうど}, and I'm sure there are others. – Aeon Akechi Mar 22 '17 at 1:55
  • @Nothingatall I guess Hepburn romanisation doesn't count these as morpheme/word boundary, although they should be. All the examples you mention come from a sound shift of hito. japanese.stackexchange.com/q/6581/1628 – Earthliŋ Mar 22 '17 at 6:28
  • I was aware of that, but they're still pronounced with a long vowel, not like 思う. – Aeon Akechi Mar 22 '17 at 12:59

Most of the time it is おう In rare cases, it is おお examples 遠く、通る おお also used for the kanji 大 or 多 , so 大きい、多い

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    Another couple of exceptions which threw me at first: 氷【こおり】and 十【とお】日【か】 – José Figueroa-O'Farrill Jul 4 '17 at 22:26

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