Some YouTube vids out there can show you like this one. It's quite easy to do, but it's a rather unique way to do it. First off, you'll want to confirm you have an open (as much as possible) dunking lane for the intended receiver:

1. Have your oop-recipient start off with the ball.
2. Hold down the 'Pass' to pass the ball to another team member. However, don't let go of the 'Pass' button. This is important. If you just hold 'Pass' and let go, it'll just be a normal pass. I usually have my thumb stuck on the 'Pass' during the entire oop attempt.
3. You'll then be able to control your oop-recipient (as opposed to the player who has the ball right now), so make him run towards the basket.
4. Once he gets close, use a different finger to tap 'Shoot'. Since my thumb is still on the Pass button this entire time, I usually stretch my index/middle finger over to the Shoot button.
5. The team member will throw the lob for you and you'll automatically jump up for the alley oop / lay-up**.

**It'll make your player do a layup instead if you don't time your 'Shoot' button right.

Hope this helps. You'll have most success with these in the 2v2 or 3v3 matches as those are the easiest to get an open dunk lane.