1. What does it mean?
ㅇㅋ is shorthand for 오케이, or its casual variant 오키. Use it to acknowledge a plan, accept a suggestion, or indicate that you have understood a message.
Also written or said as 오키, 오케이, ㅇㅋㅇㅋ, okei
2. The nuance
It is much more compressed than a full polite reply. Agreement in a friendly chat does not make the same spelling appropriate for a customer, teacher, or formal request. It also need not express excitement.
3. Examples
“ㅇㅋ, 그럼 정문 말고 옆문에서 보자.”
OK, then let's meet at the side entrance instead of the main one.
“사진은 내일 보내 줄게. ㅇㅋ, 편할 때 보내.”
I'll send the photos tomorrow. OK, send them whenever works for you.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.