How does one transcribe the romaji "ō"?

Question

I use The Kodansha Kanji Learner's Dictionary, which describes pronunciation in romaji instead of kana. I never learned romaji in a classroom, and generally only use it when reading this dictionary.

When "ō" is not proceeded by a consonant (like "tō" is), how does one know how transcribe it into kana?

I first used "おう" (such as "tō": "とう"); however, through trial and error, I came to realize that "おお" is more accurate. Then, I came across the kanji 扇 in my dictionary as "ōgi", and realized that, if I typed "おおぎ" or "おうぎ", both displayed the option of 扇.

It's quite possible this is a "spellcheck" type safeguard (the Japanese equivalent of "the" being typed out when "teh" is entered) because I have checked around various online dictionaries (Wiktionary, Yahoo!dict) and it turns out that in this case, 「おうぎ」 is indeed the correct transcription. Is there a definitive way of knowing in other cases, solely by reading the romaji?

Answer 1

It depends. In most cases it is おう. But is some words, the "おお" form is retained, such as "大【おお】きい", "多【おお】い", "遠【とお】い", etc.

For 扇, I'd believe if the dictionary doesn't have おおぎ, it should be incorrect.

(btw, from the transcription of おうぎ in classic Japanese (あふぎ) which is shown in the dictionary, the transcription now can only be おうぎ.)

Answer 2

If it's a kanji, and it's an ON reading of the kanji, it's almost certainly オウ.

Ex. Tōkyō = とうきょう

With kun readings, you just have to know which one. I don't think it's an especially common occurrence, so knowing the common cases (おおきい, おおい、とおる）should be pretty sufficient.

Even if you get it wrong, you'll be understood.

Answer 3

You seem to be considering the relation between the written and the spoken forms in the opposite way. Indeed, the writing system of a language always appears after the spoken form to transcribe it. But that is true only at the beginning. As time passes, the spoken form undergoes language change faster than the written form. As a result, when you see a discrepancy between the written and the spoken forms, the written form is not the (exact) transcription of the spoken form any more. It is the opposite: the spoken form is rather the (rough) reflection of the written form (which is tied to the original spoken form). There are both words that are written in kana as おう ([王]{おう}[位]{い}) and おお ([多]{おお}い). They are the underlying forms. They converge into the same pronunciation "ō" because of the phonological rules. It is not the other way around. There is no mapping from the pronunciation "ō" to the written forms おう or おお. But the mapping from the written forms おう or おお to the pronunciation "ō" is simple: ou →　ō.