oh

Etymology 1

Inherited from Middle English o.

intj

1. Expression of surprise.
   Oh! I didn't see you there.
2. Expression of wonder, amazement, or awe.
   Oh, wow! That's amazing.
3. Expression of understanding, affirmation, recognition, or realization.
   Oh, so that's how it works.
4. A word to precede an offhand or annoyed remark.
   Oh, leave me alone.
5. A word to precede an added comment or afterthought.
   Oh, and don't forget your coat.
6. An invocation or address (similar to the vocative in languages with noun declension), often with a term of endearment.
   Oh, gosh
   Oh baby, baby, how was I supposed to know / That something wasn't right here? 1998, Max Martin, ...Baby One More Time (song performed by Britney Spears)
7. Exclamation for drama or emphasis (often poetic).
   Oh, when will it end?
   And oh how stingingly acute, and pungently grievous and tormentive, are the remembrancing Reflections of a separate uncloathed Soul in the other World, upon a review of its mad Choice, foolish Hopes, fruitless Desires […] 1703, Lawrence Smith, The Evidence of Things Not Seen, page 143
8. Expression of pain. See ouch.
   Oh! That hurt.
9. Space filler or extra syllable, especially in (popular) music.
   I'm off with the raggle-taggle gypsy-oh. 1968, MacKinlay Kantor, Beauty Beast
10. (interrogative) Expression of mild scepticism.
    "You should watch where you're going!" "Oh?"
11. A word to mark a spoken phrase as imaginary.
    What if he says "Oh, I need to see your ID"?

noun

1. An utterance of oh; a spoken expression of surprise, acknowledgement, etc.
   There were ohs and ahs, and the people twisted about as they looked for her. Then they began to applaud. 2011, Seabert Parsons, The Lost Codex of Palenque, page 240

verb

1. (intransitive) To utter the interjection oh; to express surprise, etc.
   A quarter of an hour elapsed, and then, after several rings at the door-bell, a smothered laugh, and a good deal of ohing and ahing, the door was thrown open, and one by one, as they were announced, in came the expected characters. 1852, Merry's museum and Parley's magazine, volumes 23-24, page 46

Etymology 2

From Middle English o, oo, from Old English ō, from Latin ō.

noun

1. The name of the Latin-script letter O.
   One genuine recycled local glass of aitch-two-oh 2006, Ben Bova, Titan, page 33
   Exes and Ohs: A Downtown Girl's (Mostly Awkward) Tales of Love, Lust, Revenge, and a Little Facebook Stalking 2011, Shallon Lester, (Please provide the book title or journal name)

Etymology 3

From o (“zero”).

noun

1. the digit 0 (especially in representations of speech)
   My telephone number is four-double-three-two-oh-nine.