It was a short way of writing a different spelling of the words all correct. Some foreign-born people wrote all correct as o-l-l k-o-r-r-e-c-t, and used the letters O.K.
VOA: special.2010.02.28
But spell checkers only check that thewords being used are in fact correctly spelt: they do not check that they are thecorrectwords.
At 13 I remember being taken to a terrifying woman who made me repeat my symptoms again and again until I used thecorrect anatomical words (pretty embarrassing considering where the rash started...).