Okay

It seems more common these days to see "okay" spelled out, in place of the abbreviation "O. K." Most likely this is because it is actually fewer keystrokes (assuming the proper use of capitalization, punctuation and spacing; while the simpler "ok" can be seen in very informal writing, it would probably be seen as unacceptable in even slightly formal circumstances). It provides an interesting example of how slang develops quite rapidly. According to the AHD, "O. K." ( defined as an adjective as "1. Acceptable or satisfactory, 2. Mediocre, 3. In proper working order, 4. Correct, 5. Healthy or uninjured," as an adverb as "1. Used to express approval or agreement, 2. Well enough, adequately" and as a noun as "Approval, agreement.") was originally derived from a slang misspelling: "Oll Korrect", instead of "All Correct." What is interesting is that the original slang is now completely lost; almost no one in this day and age would think of writing "oll korrect," and now even the abbreviation is less common than its phoenetic spelling. In the next hundred years, the fact that it was ever an abbreviation at all may be largely forgotton as well.