Okay/O.K. has an unexpectedly colorful history for such a simple word, springing so often from my lips and fingertips. It first appears in the northeast U.S. in the early-middle 19th century. Mostly the fault of journalists and politicians (surprise, surprise), and their wordplay antics, this word spread through our culture to eventually infect me and everyone I know.
According to the Wikipedia article on the subject, okay comes from “oll korrect,” a conscious misspelling of “all correct,” coined by Boston journalists in the late 19th century.