In a bowl, mix together the orange juice, pineapple juice, garlic, oregano, cumin and chili powder.
Add the pork and stir to coat in the marinade. Cover and refrigerate for at least 1 hour or overnight.
Heat a large skillet over medium-high heat. Add the marinated pork and cook until browned and cooked through, about 10 minutes.
To assemble tacos: warm tortillas on a hot skillet or in the microwave. Place some of the cooked pork onto each tortilla and top with diced onion, cilantro, lime wedges and salsa.