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.

What does KO stand for?

Is it because KO and chaos are both pronounced /ka.o/?

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.