I think that not all "long o's" ( ô ) are transcribed as "ou" in hiragana, but also as "oo". However, please don't quote me on that and wait for the nihongo experts.
Usually a long "o" is spelled "ou", but sometimes it's "oo", like in ookii 大きい (big) or toori 通り (road, avenue). To avoid confusion, you can also pronounce "oo" as two separate "o", but in conversation both "ou" and "oo" sound the same.