I have read that both おう and おー are read as "ō".
When to use each one? Is there a rule for it, or is it totally random?
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Could it be that おー is written in katakana as オー? – Anna Feb 24 '16 at 13:45
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90 % of the time, you want おう (and こう etc), the rest is おお. おお never happens with an on reading AFAIK. – oals Feb 24 '16 at 15:01
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Related: japanese.stackexchange.com/q/21272/7810 – broken laptop Feb 25 '16 at 3:53
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Yes, おう in [王様]{おうさま} is read as ō, but [追]{お}う, [負]{お}う are read as ou... no? – Chocolate♦ Jun 11 '16 at 10:24
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The one that surprised me a lot when I first ran into it was 炎（ほのお）. – WeirdlyCheezy Jun 13 '16 at 17:35
In Hiragana you would use 「おう」 or 「おお」, in Katakana you would use 「オー」.
All of them can be read as "ō".
For example:
フォーク (fōku)
おおきい (ōkii)
こうえん （kōen）
