"Oh wow! A beautiful museum!"
Translation:Oho! Kaunis museo!
It should be noted that words don't have intrinsic meanings, but rather they are associated with certain concepts, connotations, intended effects and such, all of it subject to situational and dialectal variation. This is to say that it would often be too simplistic to assume an utterance has only possible meaning and is perfectly equivalent to an utterance in another language, and that there are no other utterances that could be more fitting in other contexts or equally fitting alternatives in the same context. "Oho" generally expresses one of the following: surprise, amazement, being impressed, and acknowledging a mistake or an accident. Therefore, possible translations for it that I can think of off the top of my head are "whoa", "wow", "oh", "oh my", "oh wow", "whoops", and "oops". Note that even though "oho" can translate to these utterances, it does not mean that these utterances can't translate to other Finnish utterances than "oho". Context is crucial in determining whether a translation is fitting.
I could say "Oho! Onpa kaunis museo!" if I thought the museum looked boring from the outside but it's actually beautiful when I go inside. "Oho!" expresses surprise. If already the outside was beautiful, I'd just say "Onpa kaunis museo!" ("Onpa --!" is close to "What a --!")
Of course, I'd say "oho!" when I notice an accident too (sitting on my neighbor's coat, stepping on someone's toes or dropping something, seeing a car take a sudden turn, or showing the wrong slides during presentation etc.). In addition, I use it when a child is showing off his/her new skill or how their new toy works, or a pretty good drawing. ("Oho! Piirsitkö tuon ihan itse?" Oh wow! Did you draw that by yourself?) Also when my friend looks different from usual when we meet, I might ask "Oho! Mitä on tapahtunut?" (Oh...! What has happened?) It doesn't matter if the change/surprise is positive or negative, I'd use Oho! anyway.
I've been wondering about the difference between oho, ohoh and ohhoh. They're almost the same but not exactly.
It very much depends of the situation: whether the surprise is positive or negative, whether it expresses delight, anger, disappointment or pure surprise, whether you can correct the mistake immediately, whether you think it really matters, and ultimately, the personality and company of the person saying it. Some people say "ohhoh" when sitting down on a soft seat, but I don't know why exactly. Is it like saying "Finally!" or maybe they say it because it's a small struggle to sit on such a low seat? But I really think "ohhoh" is more common when the person is disappointed in the situation. It's almost like "Hohhoijaa!" Maybe.