What this means: ㅇㅊ ?
Of the short form of Hangul I know these:

ㅇㅇ [응] – Yes.
ㅉㅉ – tsk tsk (that clicking sound you do with your tongue when you are displeased with someone).
ㅇㅋ [오키] – Okay.
ㄱㅅ [감사] – Thank you.
ㅊㅋ [축하] – Congratulations.
ㄴㄴ [노노] – No.
ㅈㅅ [죄송] – Sorry.
ㄱㄷ [기다려] – Wait.
ㄱㄱ [고고] – Gogo.
ㅂㅂ [바이 바이] – Bye Bye.
ㄷㄷ [덜덜] – Trembling
ㅅㄱ [수고] – Well done / Good job.
ㅊㅊ = Congrats
ㅗㅗ = F*ck you
ㅡㅡ - WTF
ㅁㅊ - Crazy
ㅎㄹ - Daebak
짱 - Best, Top of top or Super
etc...

Question

What this means: ㅇㅊ ?
Of the short form of Hangul I know these:

ㅇㅇ [응] – Yes.
ㅉㅉ – tsk tsk (that clicking sound you do with your tongue when you are displeased with someone).
ㅇㅋ [오키] – Okay.
ㄱㅅ [감사] – Thank you.
ㅊㅋ [축하] –  Congratulations.
ㄴㄴ [노노] – No.
ㅈㅅ [죄송] – Sorry.
ㄱㄷ [기다려] – Wait.
ㄱㄱ [고고] – Gogo.
ㅂㅂ [바이 바이] – Bye Bye.
ㄷㄷ [덜덜] – Trembling
ㅅㄱ [수고] – Well done / Good job.
ㅊㅊ = Congrats 
ㅗㅗ = F*ck you 
ㅡㅡ - WTF
ㅁㅊ - Crazy 
ㅎㄹ - Daebak 
짱 - Best, Top of top or Super
etc...

Answer

ㅇㅊ can be mean several words depending on the context.

엠창 https://namu.wiki/w/%EC%97%A0%EC%B0%BD
여친 : 여자친구
여초 : 여자초과
워추 : 워마드 개념글 추천
엄청
아침
...

Answer

I think it is ㅇㅈ
I have never heard of ㅇㅊ
ㅇㅈ means 인정 meaning I admit it.