Arrange the fish fillets in an oven proof 9x13 glass baking dish. Pour the rum and lime juice over the fish and place a slice onion on each fillet. Cover and refrigerate for 2 to 4 hours.
Preheat oven to 325 degrees F (165 degrees C).
Remove fish from the refrigerator and pour off about 3/4 of the liquid. Leave the onion slices in the dish and place a thick slice of lemon on each fish fillet. Sprinkle with oregano and black pepper to taste. Place a pat of butter or margarine on each fillet.
Bake, covered, at 350 degrees F (165 degrees C) for about 20 to 30 minutes or until fish flakes easily with a fork. Be careful not to over cook the fish or it will be dry. Serve with the cooked onion and lemon slices.