the behaviour you draw attention to is widespread and seems to be based on a response which is to some extent involuntary. so it is not unreasonable to wonder if there is some genetically mediated neural substrate. obvious questions arise re the developmental aspect, particularly in the early years, and re the presence or absence of this response in other mammalian, particularly primate, species. (it might also be worth looking at how this response is incorporated into signing languages.)
the articulation of an "Oh!" is a rapid exhalation of breath with no consonant which suggests classification with other paralinguistic vocalizations. the expression "Wow!" is similar, but the muscular accommodation required to pronounce the initial "W" suggests an element of cognitive mediation has modified the more instinctive "Oh" to add a semantic component. thus we observe that "Wow!" can be modified in quite flexible ways, by varying stress and duration.
it may help to relate "Oh!" to certain other behaviours. for example the "Oof!" of pain ("with "Ouch!" as a more formalized modification analogous to the shift from "Oh!" to "Wow!")
there is also a related form of expression eliciting a rapid intake of the breath, perhaps through slightly clenched jaw and pursed lips, which is evoked by certain over-intense stimuli - e.g. a drink which is too hot.
as a coffee-addict i have confirmed with others similarly afflicted, that the first caffeine hit of the day can invoke an in voluntary, deeply-satisfied "Ah!". this response (although it can be exaggerated for dramatic or comic effect) feels to me to be instinct-based. it does not seem to be initiated by any imperative to communicaticate. indeed the presence of another person might cause an attempt to stifle or minimize the "Ah!" (politesse)
finally one should mention in this context the now-widespread meme of the "Aha!" experience, though noting that for at least one great creative scientist, the Muse of history has recorded his "Aha!" response as the phonetically complex and semantically-loaded "Efrika!" - accompanied by an activation of the non-verbal jumping-out-of-a-bath alarm reflex.