Preheat the oven to 250 degrees F. Lay the cod in a shallow pan or baking dish. Combine the zest of 1 orange and 1 lemon, juice from the orange and lemon you just zested, and 1/4 cup of olive oil. Pour over the fish. Cover and let marinate for at least 30 minutes.
In a large oven safe skillet, heat the remaining 1/4 cup olive oil until hot. Carefully shake the excess marinade from the fish, saving it, and drop the fish into the pan. Season the fish with some salt and black pepper. Cook over medium heat for about 4 minutes, and then flip and repeat on the other side. Place the fish into the oven to keep warm while you make the sauce.
In another smaller pan, melt butter, and add the brown sugar and remaining leftover marinade from the fish. Slice the remaining orange in half and squeeze the juice into the pan. Bring to a boil, whisking regularly until the sauce has thickened slightly and reduced by almost half, about 7 minutes. The sauce should coat the spoon.
Add about 1/4 cup of finely shredded basil leaves to the sauce. Taste and add salt and pepper, if necessary. Bring the fish from the oven and place on your plate. Spoon some of the thickened sauce over it. Add more basil if desired.