The 'OK' sign originated from a 19th-century joke.
Why it is true
In the 1830s, young people in Boston created the expression 'oll korrect' as a joke on the spelling of 'all correct'. The abbreviation 'OK' became popular during the 1840 presidential campaign.
Source: Linguistics
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.