Is OK an expression from the Civil War meaning 0 killed?

Answer 1

No, the expression predates the War. One theory is that its a leftover from the 1836 presidential campaign. Martin Van Buren was the Democratic candidate, from Kinderhook, New York, called "Old Kinderhook", OK for short. Another theory is that a few years before that, there was a group of young dandies, men about town in New York. They thought they were terribly witty, and one thing they liked to do as a joke when writing to each other was to deliberately misspell words, for comic effect. One such mangled expression was "Oll Korrekt" (all correct), soon abbreviated OK.