"How is the weather in Kaunakakai?"
Translation:Pehea ke anilā i Kaunakakai?
The particle ”o” (and also “a”, but you don't need to worry about the distinction between them yet) is the genitive, whereas “i” is the preposition “in”.
“Pehea ke anilā i Kaunakakai i kēia lā?” = “How is the weather in Kaunakakai today?”
“Pehea ke anilā o Kaunakakai i kēia lā?” = How is Kaunakakai's weather (the weather of Kaunakakai) today?”