Exactly. Just like a dog does not say "bark" but "woof". 'Bark' is the just the name for the noise made, the onomatopoeia is 'woof' (sometimes also 'bow wow'). Although for the owl noise, I've only ever heard it doubled up, like '(w)hoohoo'. All my variations were marked wrong which I've reported.
Edit: I found a list of cross-linguistic onomatopoeia here, which shows twit twoo/toowit toowoo in addition to hoo hoo.