It's not unusual in France to use OK and KO. I never liked it much because it's too easy to confuse and no semantically a good opposite either, but visually it does look very balanced.
I've seen it a lot in Spain as well. I guess it's a combination of just being the reverse of OK, and also "KO" meaning "knocked out" just feels like the opposite of OK.
I've always just understood it as in knockout, chaos wouldn't fit in the way OK and KO are used (as predicate adjectives, if I understand English grammar classes correctly) but it is part of why some people like to use "KO", I'm not sure.
The origin of OK is disputed; however, most modern reference works hold that
it originated around Boston as part of a fad in the late 1830s of
abbreviating misspellings – that it is an initialism of "oll korrect" as a
misspelling of "all correct". This origin was first described by linguist
Allen Walker Read in the 1960s.
