Oh Ji is Korean for "five fingers" and refers to a striking technique that uses the fingertips drawn together into a single mass. It is sometimes alternately called a Crane's Beak as the hand and wrist, when in this position, somewhat resemble the head of this bird (see link) and the striking surface would correspond to its beak/bill. Power is manifested by a loose and flexible wrist (unlike the rigid wrist necessary for success with a Straight Punch), and calls for a whipping or snapping action by the forearm to deliver a penetrating blow.
This striking technique is considered advanced since much practice is needed to attain the quick arm movement and rigid finger position, while keeping the wrist loose and supple. Usual targets tend to be soft, sensitive areas of the body, as well as easily accessible pressure points.