Is fair enough completely equivalent to OK? If not, under what circumstances should I say fair enough in my response? Any good examples will be most welcomed.
No, 'fair enough' is not completely equivalent to 'OK'.
'OK' means many things and is very frequent. It can mean 'I agree' or 'I accept what you say' or 'Yes' or similar affirmatory things.
'fair enough' means a (non-confrontational) argument-disagreement is being conducted, and you are conceding a point (which is affirmatory).
"How are you feeling?" "OK."
"I'm not broke." "You did overdraw your account" "Fair enough"
Switching the two would not work in either case.
Ideally fair enough is used when you agree partially. To quote from Chambers,
"Expressing acceptance, though not necessarily full agreement"
But the general usage likens fair enough to OK. There are some example usages at Wordnik.