In a blender or food processor, combine orange juice (1/2 cup), lime juice (1/4 cup), 3 tablespoons oil, salt (2 teaspoons), garlic (3 cloves), vinegar (1 tablespoon), oregano (1 1/2 teaspoons), cumin (1 teaspoon), pepper (3/4 teaspoon), achiote powder (1 1/2 ounces), smoked paprika (1 teaspoon), and onion powder (1 teaspoon). Blend until smooth. (The paste may separate from the citrus juice.)
Place chicken (2 pounds) in a large bowl or zip-top bag. Pour the marinade over the chicken and toss to coat. Cover and refrigerate for at least 2 hours, preferably overnight.
Preheat the grill or grill pan to medium-high (about 350°F). Brush the grates with vegetable oil. Grill chicken in batches for 3-5 minutes per side or until the internal temperature reaches 165–175°F (74-79ºC). Let rest for 5 minutes before slicing or chopping.
Warm corn tortillas on the grill or in a dry skillet until soft and lightly charred. Keep warm, wrapped in a clean towel.
Assemble tacos by layering grilled chicken into tortillas. Top with chopped onion and cilantro. Serve with lime wedges and any desired optional toppings.