Mix the olive oil, orange juice, thyme, salt and pepper together in a shallow bowl.
Add the mackerel and marinate for 20 minutes.
Preheat the broiler.
Placethe mackerel on a broiling or baking pan and brush with the marinade.
Broil about 6 inches from the heat source, basting once or twice for 5 to 8 minutes, until the fish is lightly browned and almost cooked through (it will be tender and opaque throughout).
Top with the oranges, baste once more, and broil for another minute.
Serve immediately.