OK

In the late 1830s, American newspapers ran a fashion for misspelled abbreviations. 'KG' for 'no go' (know go). 'OW' for 'all right' (oll wright). 'NS' for 'enough said' (nuff said). It was the era's bad pun. The Boston Morning Post on March 23, 1839, used 'o.k.' as shorthand for 'oll korrect'. The joke should have died with the rest of them.

Then Martin Van Buren ran for re-election in 1840. He grew up in a New York town called Kinderhook and his Democratic supporters formed clubs they named O.K. Clubs (Old Kinderhook). Banners and posters everywhere read O.K. The campaign joke amplified the newspaper joke. Van Buren lost the election. The abbreviation survived him.

The decisive accident was the telegraph. Operators needed short, unambiguous codes to confirm receipt of a message. O.K. was two letters that sounded like nothing else in the traffic, and hard to mishear as anything. The 1840s commercial telegraph network drilled it into business English.

From there it spread by the usual American export channels: merchants, movies, occupation forces. What makes it unusual among loanwords is that it traveled without being translated. Most borrowed words get reshaped by the mouth that takes them. OK arrived as itself and stayed that way.

There is no equivalent journey for any other word. 'Hello' is younger and stayed mostly inside English. 'Coca-Cola' is a brand, protected and marketed. OK belongs to no one. It entered language after language as a full word, with nobody asking it to behave.