In this comic there's a long "ooooooh" at the end.
How can I tell if it's a long "oh" (/o/) or a long "oo" (/uː/)?
I thought it could be the 'h', but I don't think I've never seen "ooooooo" anywhere.
How does a native speaker read this sound effect, and how would you write a long "oh" and a long "oo"?
Since neither "oooooh" nor "ohhhhh" are words that can be looked up in a dictionary, only a general rule of thumb can be given. That rule of thumb is that if there is more than one O, then it is [u:]. If there is only one O and one or more H's, then it is [o]. So in your example:
ooooooh
It's [u:].
ohhhhhh
This is [o].
If there are multiple O's and H's then it's [u:].
I don't think there is a hard and fast rule for that. I guess it depends on context to some extent. The comic isn't particularly helpful in that regard. I don't know what the artist intended either. Aside from the spelling suggested by the other answers here, there's a suggestion that "o" would be more to convey surprise or shock while "u" could convey disapproval or foreboding... But that's not absolute either. This is a little like asking how to pronounce something that is misspelled. Not necessarily a shortcoming of the reader but of the author/artist.
