Technically "good" would imply something a little better than "Okay," but it's a very fine line of difference. You would get more information from paying attention to tone of voice or facial expression. The words (in normal conversation) are practically equal. An example of tone of voice, etc. A person could say "I feel great," but say the word "great" with a sarcastic voice, meaning "not great at all," or they could roll their eyes when they say it, to imply the opposite of what they say.

'I'm good' is not technically speaking grammatically correct. You would say, 'well thank you'. I think that the Americans use it, but I am not sure. I actually think that a sub-group of South Africans use it as they watch many American movies, but English is not their native tongue. 'I'm ok' tends to mean that you are just hanging on, at times. Other times, you are well, but not overly so. You would need to check tone and body language to discern the meaning.

"I am okay" and "I am good" aren't exactly the same. "I am okay" is a lot more neutral (and less positive) than "I am good". "I am okay" is almost like saying "I am good, but ..." or "I am good but I could be better". To give you an idea of the difference between the two, if '0' was feeling bad and '10' feeling great, then "I am okay" generally implies you are somewhere between 4-6, and "I am good" is 6 or more.