I've never seen IMAO before, but Google says it means "In My Arrogant Opinion". I have seen IMHO many times. It can mean My Humble Opinion and it can mean In My Honest Opinion. All of these are a way of saying "I think".

Yes, they are acronyms. The answers below are correct! :)

'IMHO' es 'in my ['honest' o 'humble'] opinion...', y es lo más común. 'IMAO' podría ser 'in my actual opinion'—pero no estoy seguro de 100%.