Why do we say, "Oh!" when we are surprised? I have noticed that I and many of my peers tend to say "Oh!" whenever we are surprised or think of an answer to a question. This "Oh!" is not only prevalent in English, but also in Korean and German. 

Edit: Sorry for not clarifying. I was wondering why we say, "Oh!" instead of perhaps, "Pft!" or "Tsch!" or any other type of sound.
This is a hard question because there are probably many different (but not equally valid) answers.

From my perspective (Barrett &amp; Russell, 2015), there are a couple of things to think about.


How often do you actually say "Oh!" when you are surprised? Your memory of these instances is likely unreliable, as you're depending on semantic memory about cultural beliefs, and not on episodic memory of your actual behavior (Robinson &amp; Clore, 2002).  Indeed, we might expect a lot of variability across different contexts (see next bulletpoint).
How much variability is there within/between individuals and cultures?  If we've learned anything from relating emotions to variables like physiology, behavior, and facial expressions, we know that there is no one-to-one correspondence (e.g., fear and running away, anger and increased heart rate, etc.).  Instead the relations between discrete emotions and these response systems change across individuals, contexts, and cultures.
Does surprise produce an "Oh!" or does an "Oh!" indicate surprise?  On the one hand, you can think of an emotion as a reaction to some stimulus, producing loosely coordinated responses (e.g., "Oh!").  On the other hand, you can think of emotions as interpretations or categorizations of what's going internally and externally.  In this sense, you infer from your "Oh!" that you are surprised.  This means that "surprise" is a category that you deploy for all the instances in which you say "Oh!"  This is sort of like how "bird" is a category for all instances in which the object being described is animate and has wings.


So in a tautological way, you say "Oh!" when you are surprised because you categorize all instances of "Oh!" as surprise.  Either way, your question has not been studied empirically (to my knowledge), so it's hard to give an answer based on actual data, and different theories will make different hypotheses.