Whisk together 1/4 cup of the oil, 2 tablespoons of the thyme, ancho powder and chopped garlic in a small bowl. Skewer the shrimp and brush with marinade.
Place the remaining 1/2 cup of the oil in a small saucepan, add the sliced garlic and cook until the sliced garlic is lightly golden brown. Remove the garlic slices with a slotted spoon to a plate lined with paper towels. Reserve the oil.
Increase the heat of the grill to high.
Remove the shrimp from the marinade, season with salt and pepper and grill until golden brown on each side, about 1 1/2 minutes per side. Remove the shrimp from the skewers, transfer to a platter and drizzle with some of the reserved garlic oil and the garlic chips. Sprinkle with the remaining thyme and garnish with oregano leaves.