Well, I believe most Chinese understand OK as more and more young people are using the word in the daily life.

But it is still good to know how to say it in case someone may say it to you. The equivalent word for OK in Chinese is 行. It is prounced as xíng, the second tone.

When you want to say 'Not OK', you can say '不行' - bú xíng
不(bú) means not or no, a useful word to learn.