ありがとう is always the only acceptable way of writing it.
ありがと or ありがとお might be used in manga but it's just for fun or to make it funny basing on the fact that everyone knows ありがとう is the correct one.
I'm assuming its like in English when we write things like " I gots me a candy barrrr"
...
Actually we do that a lot, and some people talk like that... poor souls.
^_^
I don't think so. It's ありがとう, and can be shortened to ありがと, but not ありがとお. It's more like when people spell things like "sentance" or "definately" or "grammer".