In modern English, there isn't much difference. Perhaps "it's ok" is more informal than "it's fine", but they're generally interchangeable.

Just as Jude said they are generally interchangeable meaning you can use either one of them and the sentence would have the same meaning. I will give you examples. Example 1) I told my friend that it's okay to make mistakes. Example 2) I told my friend that it's fine to make mistakes. I hope this helps!