Fun fact: The correct spelling is "O.K."*. I learned this when I got it wrong on a spelling test that I didn't study for because I was awesome at spelling. However, since the vast majority of people assume that the two-letter version is the wrong version, I grudgingly spell out "okay" so they don't put me in the same category as people who confuse their homonyms**. * According to wikipedia, the etymology is contested such that it seems "okay" might be legit as well, in which case I want my grade corrected. ** Not that I mind when people do confuse their homonyms -- it's more annoying when people make lists of grammar mistakes they hate. My sister and brother-in-law both respond to text messages with "K", and it doesn't bother me, but it can be confusing as to whether I've annoyed them or if they're just trying to indicate that they have read my message.