Is it Okay to Use O.K. in Historical Fiction?

When did the term "O.K." or "okay" find its way into common use in American English? Is it incorrect (and inaccurate) to use "okay" in 19th century fiction? What if it's spelled "O.K."? What does O.K. stand for, anyway? I've provided numerous historical newspaper articles and snippets showing the etymology and proving one of the spellings (O.K.) is highly accurate in the 19th century, but the other (okay) is not.