Am I correct assuming that both おお and おう are homophones in Japanese - both being pronounced as long o? I suspect I am, since that's what I'm indirectly reading in various tutorials. If so, are there any (non necessarily exhaustive) rules to guess the correct spelling of the words with long o? I speak several languages where there are no strict spelling rules, but still there are some hints that help guess the right spelling most of the time. Is there such a rule for おお vs おう in Japanese?

  • 2
    Volitional form of よそおう → よそおおう. Volitional form of おおう → おおおう.
    – istrasci
    Commented Sep 17, 2013 at 14:45

5 Answers 5


Etymology. おお comes from an earlier おほ or おを, while おう can come from any of おう、あう、おふ、or あふ (and potentially えう、えふ、ゑう、ゑふ if it's now よう). This is due to sound change - originally all of these were distinct pronunciations, but they have since been reduced to a single sound ([o:]).

Typically you can guess that [o:] in Chinese loanwords will be spelled with おう (since neither おほ nor おを ever occurred in any Chinese loanwords), and that in native Japanese words it will be spelled with おお (since the combinations that led to おう were fairly rare compared to おほ and おを, though they do occur).

  • But Tōkyō is written as とうきょう, so is it not a native Japanese word?
    – Ankit Seth
    Commented Jul 13, 2018 at 17:49
  • 1
    @AnkitSeth It's composed of parts loaned from Chinese (so it's 'kango'). If you wanted to make it purely Japanese ('Yamato-kotoba'), it would be something like 'Higashi-no-Miyako'.
    – Sjiveru
    Commented Jul 13, 2018 at 19:10
  • So should it be pronounced as to-u-kyo-u or to-o-kyo-o ? (u = う)
    – Ankit Seth
    Commented Jul 14, 2018 at 7:19
  • 1
    @AnkitSeth The second. おう in a single character's on'yomi is universally /oo/.
    – Sjiveru
    Commented Jul 15, 2018 at 19:48

It reflects old kana usage. Basically, where there's おお, the second お used to be either ほ or を。 There's some discussion here, with the example of 通り (used to be とほり、now とおり) vs 党利 (used to be たうり, now とうり).

Reading that site also suggests that they're not strict homophones, but the modern pronunciation is close enough that writing そのとうり instead of そのとおり is a common error (it seems to be the classic example used whenever I've seen this issue discussed).

  • 3
    I don't think there really is a difference in pronunciation, small or otherwise. I think that claim needs some kind of evidence.
    – user1478
    Commented Sep 16, 2013 at 18:10
  • 1
    I'm going off the quote in the link: それが現代では、「通り」も「党利」も、同じように「とーり」と発音することが可能となりました。しかし厳密な発音は、全く同じとは思えません
    – nkjt
    Commented Sep 16, 2013 at 21:27

Both おお and おう spell the same sound in modern Japanese, namely the long vowel /oː/.

The choice of spelling is etymological, but there is one rule of thumb: /oː/ spelled as おお can only occur inside a native Japanese word or be split between two morphemes. Unfortunately, the converse is not true: /oː/ inside a native Japanese word can be spelled おう, e.g. 扇【おうぎ】, and it is possible for /oː/ split between two morphemes to be spelled おう as well, e.g. 小道【こうじ】. It should go without saying that two /o/ in a row, i.e. /o.o/, is always spelled おお.

In terms of history, modern おお can only come from old おお, おほ, or おを; all others (e.g. あう, あふ, おふ) became おう. There are some irregular derivations as well, e.g. 妹【いもうと】 ← いもひと, 小道【こうじ】 ← こみち, 申【もう】す ← まをす, and even こうぶり ← かがふり!

It is worth noting that Christian sources show that Late Middle Japanese had a phonemic distinction between おお・おほ・おを /owo/ and おう・おふ /oː/ (including えう・えふ /joː/) and あう・あふ /ɔː/ (including やう・やふ /jɔː/); for instance, 十【とを】 is recorded as touo. All three have merged in modern Japanese.


Basically it is something to learn, be aware of and not dwell on too much.

The pronunciation can be different. I have one example from my notes:








You might be able to look these up on a modern work tank, the NHK dictionary for pronunciation or, the only place I have found this tested (using the above expressions), Nihongo Somatome N1 Choukai, p13, practice question 2

  • 3
    Can you elaborate on what you mean by "the pronunciation can be different"?
    – user1478
    Commented Sep 16, 2013 at 22:26
  • Not being a real linguist it is difficult to give more than the above examples. I could be wrong; it may be just a matter of emphasis...looking at the NHK dictionary, this may be the case as the three words are all spelt オーイ/オオイ、オーベイ、オーハバ with differing "lines above them for emphasis"...in which case I might need to revise my answer to make this clearer (?)
    – Tim
    Commented Sep 17, 2013 at 12:49

Unfortunately, there is no particular rule. The only way to know it is to learn it...

Regarding pronunciation, I would say that they are slightly different, even if the difference is barely audible... I would not say they are strict homophones...

  • 2
    But if they are not strict homophones, then wouldn't learning to capture the (barely) audible difference constitute as a rule? Commented Sep 16, 2013 at 14:02

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .