Since they're in the same font, I'm guessing 「うおお！！」 is spoken by the same person as 「もうがまんできねぇ！！」, so "Whoa" isn't really an accurate translation.

Yeah, I had wondered about that. We could change it to "Uuhggg!!" or "Hoowah!!" or "Oowah!!" or whatever anyone thinks would sound good.

... or, we could just leave it as "Uoo!!", since, uh, that's what it is...