OK Grip is the basic posture of your hand to perform jelqing exercise.
A very basic jelqing exercise is to stretch a half-erected penis with a OK-grip from the shaft.
The idea is to push more blood through the penis, so a little force is needed when you move your OK-Grip.
However, we do not want to get a full erection. Stop jelqing when your penis is fully erected.
Some people only use Over-hand OK Grip, and some only use Under-hand OK Grip.
It is ok to use either one, or use both in a jelqing session. If you wish to learn more about penis enlargement, read the most popular beginner routine for jelqing.