う meaning ō or u?

Hello, I have been trying to move to just reading the hiragana/katana on the lessons instead of the romanji to improve my reading skill. I always see う used for ō, is it always used for that character or is it sometimes used for u? how can you tell when its which character. e.g. もう means mō not mu

Hello again, I also have this issue with other characters such as Ē. :!:

Konnichiwa Emesto-san. In Hiragana, a double-length vowel is shown by adding the appropriate vowel character. So, for example, in the ... :arrow: あ (a) hiragana column syllables: add *あ* >>> ああ = ā 　/ かあ= kā　/ さあ = sā　/ まあ = mā :arrow: い (i) column syllables: add *い* >>>> いい = ī (or sometimes spelled as 'ii') / きい = kī (or kii) / ちい = chī (or chii) / ひい = hī (or hii) :arrow: う (u) column syllables: add *う* >>>> うう = ū / くう = kū / ゆう = yū / ふう = fū :idea: The remaining two vowels are not so obvious. :arrow: え (u) column syllables: NORMALLY, add *い* (i) >>> けい = kei / れい = rei SOMETIMES add *え* (e) >>> ええ = ē / ねえ = = nē / へえ = = hē / :arrow: お(o) column syllables: NORMALLY, add *う* (u) >>>おう = ō / そう = sō / もう = mō SOMETIMES add *お*(o) >>>おお = ō / とお = tō / よお = yō

ありがとう　ございました　さやか　さん　！

You are very welcome! - Sayaka :P