The best way to state why this question/statement starts with 'O would be this -
In Hawaiian, sentences cannot start with a noun, proper noun, or pronoun by itself. When it's a proper noun, pronoun or a definite noun ( the ....) you must put 'O before it at the start of this type of sentence. Way back when, we called it a verbless equational sentence - 'O Ka'iulani = kona inoa?
In sentences with verbs, the verb is usually first. There are actor emphatic sentences with the subject starting the sentence and the verb afterwards, and you will find that a preposition will come before the noun in that case.