Japanese 1 The annoyance of "oo"

Is Shachoo spelled しゃちょお or しゃちょう? I was under the impression that if two "oo"s where present I'd have to put a う at the end but apparently that is not always the case.

Thanks fellas.

Another question when it comes to katakana how do I spell words like reezun and chaahan?

レーズン　AND　チャーハン　OR　レエズン　AND　チャハアン