In Japan they have the abbreviation “NG” (“not good”) that’s often used as the opposite of “OK”. I always liked how, when I worked for a Japanese machine learning company, classifiers would be between “OK” and “NG”. Maybe semantically “okay” and “not good” are not great opposites, but I always liked how having textual alignment makes the contrast immediately obvious

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.

Seems like “not okay” would be the cleanest antonym. I can’t see anything that could go wrong with using it.

> Meeting at 4 pm today?

> NO