When there’s a bar over a vowel the vowel is usually doubled up but not in the case of ō. Ō seems to mean either a doubled up vowel or in some cases an う. Is there a rule I can use to distinguish whether its one or the other. This isn’t an issue on Wanikani but I notice so many online learning resources as well as textbooks use this ō but it’s never clear what it represents so I can’t tell by reading it whether my pronunciation is correct.
Here’s an example where ō represents う
Father = Otōsan = おとうさん
Here’s an example where ō represents お
Big = Ōkī = おおきい