When to use おう and when to use おー?


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?




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）

