In a small bowl, combine the oregano, cumin, paprika, 2 tsp. pepper and 1/2 tsp. salt; rub over fillets.
After rubbing the fillets, refrigerate for at least 30 minutes.
Meanwhile, in a small bowl, combine the mango, avocado, red onion, cilantro, lime juice and remaining salt and pepper; chill until serving.
In a large cast-iron skillet, cook fillets in oil over medium heat until fish flakes easily with a fork, 5 - 7 minutes on each side.
Serve with salsa.