'OK' is almost certainly the most widely understood word on Earth

The word 'OK' originated as an 1839 Boston newspaper joke — an abbreviation of 'oll korrect', a deliberate misspelling of 'all correct'. It went viral when it was used in a Presidential campaign slogan and never stopped spreading. Today it is recognised and used across more languages than any other word.

Source: Allan Metcalf, 'OK: The Improbable Story of America's Greatest Word' (Oxford University Press, 2011)