I'm not quite sure what you exactly mean by 'I'm not OK'. If that is your answer to questions like 'how are you?', it could be translated as 不太好, which is equivalent to 'Not so good' in English. But beware. If you tell others 不太好, you may be asked about what happened. If you were asked 'Are you ok with sth?', they are asking you if you mind sth. Then, 'I'm OK' in that situation means 我同意/我没有意见。I'm not ok (with it) means 我不同意/不愿意/我介意.

没关系。

不太好。 心情不好。

我不怎么样。or 我一点都不好 Image this situation ,someone just offended you unconsciously ,then he said :I'm sorry ,r u okay ? The answer in chinese would be 我一点都不好，不怎么样 instead for I'm not feel good . I'm not okay

不好的意思