It seems to be a unique word, one of a kind. Pronounced as if someone asked how it’s spelled.
Rather than ‘ok’ (‘sock’ without the ‘s’ sound) it’s ‘owkay’. As far as I know it’s the only word (apart from ‘I’ if ‘I’ is a word) that has that attribute.
Etymology is the word you’re looking for.
As for the etymology of “ok”, read the words of the great Cecil and be enlightened.
When using software with ‘OK’ buttons, I often think of them as ‘ock’ buttons. 
That Cecil article on the etymology of OK is brilliant, the kind of stuff that caused me to join the SDMB, and, later, subscribe.
Am I the only one who first learned ‘OK’ as a spelt-out word ‘okay’, and only later ran into ‘OK’, and thought it was some kind of abbreviation of the longer word?
‘Ow-kay’ rather than ‘oh-kay’? How do you pronounce ‘how’? Or were you thinking of ‘flow’? I’m just wondering whether you have a very different dialect of English than me (southern Ontario).
Trivia: ‘Okay’ has made it into Esperanto as ‘okej’, pronounced the same.
As Q.E.D.'s link shows, OK is an acronym (or initialism for the pedantic) and they are pretty much always pronounced this way. eg: eg, ie, nb …
Dashiell Hammet’s detectives all say “Okey”. Just an interesting tidbit…
The Times spells it “okay”.
It isn’t unique. There are many words, or initialisms, of this kind in English. On the QT, he KO’d the guy in the third round, etc.