Oi-zuki is a compliment to the Choku-zuki and Kizami-zuki with foot movement. It is performed as lunge and thrust punch with the lead hand. In addition to the block techniques, Oi-zuki is one of the first techniques a student learns.
Oi-zuki is used as a defensive counter if you are being rushed. If an attacker is closing distance, stepping into Oi-zuki means they run into an immovable structure which ruins their attack, their own structure and balance.
A counterpart to Oi-zuki is the Gyaku-zuki (Reverse Punch).
Gyaku-zuki in another of the basic punches. The reverse punch is sometimes called a counter punch because the lead hand may be used to block an attack with the non-leading hand free to counter.
Gyaku-zuki is more powerful than Oi-zuki because power is generated by rotation of the hips with the thrust of the punch.