“WOW!” expresses surprise, much as does “Oops!” “Whoops!” expresses dismay at a thing dropped and broken. These are all onomatopoetic expressions (though the word onomatopoetic expresses nothing that I can think of – with the possible exception of a poet being pelted with rotten eggs.) “OK” is a word that came from the jocular “orl korrect”, and was not spelled “Okay”.
I suspect that North Americans popularised “Okay” — Australians don’t create expressions, we simply shorten words into more easily spoken forms: electricians into “sparkies”, carpenters into “chippies”, and so on.
Jesus taught us not add to our words but instead let them speak for themselves. Jesus also said that we should speak succinctly, our “Yes” meaning “Yes” and our “No” meaning “No” – any addition is unnecessary.
When I was a boy, I used to say “My oath”, which was not just wrong but unnecessary if I really meant what I said (I can’t say that I always did).
Then there was “Cross my heart and hope to die!” This awful exclamation signified the doom that would follow a lying statement. It died late last century, and no one mourned its passing.
When a signalman in the Australian Army, I learned Voice Procedure: the necessity of brevity of speech and economy of words when speaking over the air. “Over” meant “I’ve finished speaking, it’s your turn.” And “Out” meant “That’s all.”
“WOW!” is an expression of astonishment. I like the vowel “O” in the middle because it’s the shape our mouths take when we’re delighted. Each “W” in “WOW!” serves as a kind of bookend that holds the “O” in place. Jesus taught us not to add oaths to our statements in order to make them more believable.
Nevertheless, I don’t think our Lord would mind us exclaiming “WOW!” when he heals the sick and restores broken body parts. It would have been shorthand for his disciples and others as they witnessed his many amazing miracles.
Try to say “YUK!” without your nose heading south to meet your chin as it moves north. You can’t. “UGH!” raises the jaw but doesn’t draw down the nose. “EEOW!” is an expression that conveys distaste at the very thought of someone or something. It’s very girly and I can’t imagine a boy using it.
The thing about “WOW” is that we don’t have to work at saying it because it comes out naturally as a near-perfect response the moment we see yet another startling miracle by our awesome Lord! (Maybe with an “Oh” in front, as in “Oh, WOW!”)