First, some nomenclature. The basic osis catch is covered here. It involves catching behind the back as one spins away from the disc. A similar spinning away movement can be applied to almost any catch. In this example, Matt is showing us the flamingosis, which is a flamingo with the osis style rotation. There is also gitosis, chosis (chair osis), bad attitosis, and probably a whole host of others. So, take your favorite catch and try to add an osis to it. It can open a whole new world.
Now for the flamingosis. Set the disc up and then spin. If you spin to the left, as Matt demonstrates, then plant on your right foot, kick your left foot out, and reach behind your right leg with your right hand and make the catch. As you spin, watch over your right should as long as you can, then flip your head around quickly and watch the disc into your hand over the left shoulder. Matt points out that the motion is very similar to a phlard. So, one way to begin is by catching a pharld but move your hand to the other side of your leg. This will help you with the motion.
There are also some subtle variations here. For example, the raised leg could travel over the disc before the catch is made. Or, it could move into position before the disc is low enough for the leg to go over. Or it could never go past the disc flight path at all. All are valid but it changes the aesthetic. My guess is that, with a little creativity there are other possible variations as well. If you think of any, please share in the comments below.