How does one transcribe the romaji "ō"?


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?




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.)




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.)

