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.

>>1

don't worry too much about it; the only exceptions worth noting are おおく and おおきい・おおきな