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

This is Pieter Claesz, C-L-A-E-S-Z, probably mispronounced.

这是彼得·格莱兹的作品,C-L-A-E-S-Z,我可能发音不准确

耶鲁公开课 - 欧洲文明课程节选

It's like they spell colour, c-o-l-o-r, we spell it c-o-l-o-u-r,

这就像他们拼写颜色这个单词，c-o-l-o-r，而我们拼写成c-o-l-o-u-r，

美式英语和英式英语 - SpeakingMax英语口语达人