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
2
"But there were a couple "trouble songs" that just weren't working. They were o.k., but I wanted something different.
VOA : standard.2009.04.21
3
O.K. But don't you be thinking that I'm goingto be calling you "sir."