From Wikipedia: "An alley-oop in basketball is an offensive play in which one player throws the ball near the basket to a teammate who jumps, catches the ball in mid air and immediately dunks it."
This needs to be done in Single player with two controllers. It's also doable in multiplayer but you need at least 3 players so you can get 2 players to join the same team.
Go near the goal of your opponent and hop on to the small platform.
Now use your second controller to get the ball and hop on the bigger platform to the right of where your other player is.
Then push down

to start the pass BUT before you release it, press

on your main player to jump and when your player is in the air, release

on the other controller to complete the pass.
Now you need to kick the ball to the goal with

or

while still being in the air to complete your Alley Oop.
Here is a video made by PressA2Join.com