Here is a scenario where the difference between the two phrasings is important:
The Heimlich Maneuver and Cardio-Pulmonary Respiration (CPR) are urgent, potentially life-saving actions that are also dangerous. They should only be performed on a person who is not breathing. Furthermore, the "cardio" portion of CPR should only performed on a person whose heart is not beating.
Therefore, I was taught that the first step of the Heimlich Maneuver or CPR is to ask, "Are you OK?". The purpose is to get the person to try to answer verbally. If the person can say anything -- even a weak "no", then neither the Heimlich Maneuver nor CPR should be performed.
In a verbal conversation, the minimal formality of "Are you OK?" requests a verbal answer.
"You okay?" is so informal that it can be answered with just a gesture. In a situation where the asker needs to know if the answerer can exhale, this is too informal.