Coarsely chop 1 onion half. Coarsely chop 2 pineapple rounds, discarding core.
Save a few chopped pineapple for garnish. Place chopped onion and chopped pineapple in blender. Add orange juice, white vinegar, chili powder, minced garlic, salt, dried oregano, cumin, chilis and cilantro and puree marinade until smooth.
Light you charcoal in the KUDU and let them burn to embers with a layer of white ash.
Adjust your grill height over the embers to medium-heat. To locate medium heat, place your hand over the embers and find the height you can count to 3 or 4 before having to remove your hand from the flame (approx. 500 degrees).
Move your KUDU grill grate to that height and place your oysters on the grill.
Drizzle oysters with olive oil.
Add a spoonful of the Al Pastor sauce into each oyster shell.
Allow oyster to roast and sauce to bubble.
Remove from grill and add pineapple and cilantro garnish.
Remove and serve.